      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283
    284
    285
    286
    287
    288
    289
    290
    291
    292
    293
    294
    295
    296
    297
    298
    299
    300
    301
    302
    303
    304
    305
    306
    307
    308
    309
    310
    311
    312
    313
    314
    315
    316
    317
    318
    319
    320
    321
    322
    323
    324
    325
    326
    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    339
    340
    341
    342
    343
    344
    345
    346
    347
    348
    349
    350
    351
    352
    353
    354
    355
    356
    357
    358
    359
    360
    361
    362
    363
    364
    365
    366
    367
    368
    369
    370
    371
    372
    373
    374
    375
    376
    377
    378
    379
    380
    381
    382
    383
    384
    385
    386
    387
    388
    389
    390
    391
    392
    393
    394
    395
    396
    397
    398
    399
    400
    401
    402
    403
    404
    405
    406
    407
    408
    409
    410
    411
    412
    413
    414
    415
    416
    417
    418
    419
    420
    421
    422
    423
    424
    425
    426
    427
    428
    429
    430
    431
    432
    433
    434
    435
    436
    437
    438
    439
    440
    441
    442
    443
    444
    445
    446
    447
    448
    449
    450
    451
    452
    453
    454
    455
    456
    457
    458
    459
    460
    461
    462
    463
    464
    465
    466
    467
    468
    469
    470
    471
    472
    473
    474
    475
    476
    477
    478
    479
    480
    481
    482
    483
    484
    485
    486
    487
    488
    489
    490
    491
    492
    493
    494
    495
    496
    497
    498
    499
    500
    501
    502
    503
    504
    505
    506
    507
    508
    509
    510
    511
    512
    513
    514
    515
    516
    517
    518
    519
    520
    521
    522
    523
    524
    525
    526
    527
    528
    529
    530
    531
    532
    533
    534
    535
    536
    537
    538
    539
    540
    541
    542
    543
    544
    545
    546
    547
    548
    549
    550
    551
    552
    553
    554
    555
    556
    557
    558
    559
    560
    561
    562
    563
    564
    565
    566
    567
    568
    569
    570
    571
    572
    573
    574
    575
    576
    577
    578
    579
    580
    581
    582
    583
    584
    585
    586
    587
    588
    589
    590
    591
    592
    593
    594
    595
    596
    597
    598
    599
    600
    601
    602
    603
    604
    605
    606
    607
    608
    609
    610
    611
    612
    613
    614
    615
    616
    617
    618
    619
    620
    621
    622
    623
    624
    625
    626
    627
    628
    629
    630
    631
    632
    633
    634
    635
    636
    637
    638
    639
    640
    641
    642
    643
    644
    645
    646
    647
    648
    649
    650
    651
    652
    653
    654
    655
    656
    657
    658
    659
    660
    661
    662
    663
    664
    665
    666
    667
    668
    669
    670
    671
    672
    673
    674
    675
    676
    677
    678
    679
    680
    681
    682
    683
    684
    685
    686
    687
    688
    689
    690
    691
    692
    693
    694
    695
    696
    697
    698
    699
    700
    701
    702
    703
    704
    705
    706
    707
    708
    709
    710
    711
    712
    713
    714
    715
    716
    717
    718
    719
    720
    721
    722
    723
    724
    725
    726
    727
    728
    729
    730
    731
    732
    733
    734
    735
    736
    737
    738
    739
    740
    741
    742
    743
    744
    745
    746
    747
    748
    749
    750
    751
    752
    753
    754
    755
    756
    757
    758
    759
    760
    761
    762
    763
    764
    765
    766
    767
    768
    769
    770
    771
    772
    773
    774
    775
    776
    777
    778
    779
    780
    781
    782
    783
    784
    785
    786
    787
    788
    789
    790
    791
    792
    793
    794
    795
    796
    797
    798
    799
    800
    801
    802
    803
    804
    805
    806
    807
    808
    809
    810
    811

    //!  Decoding of DXT (S3TC) compression
    //!
    //!  DXT is an image format that supports lossy compression
    //!
    //!  # Related Links
    //!  * <https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_texture_compression_s3tc.txt> - Description of the DXT compression OpenGL extensions.
    //!
    //!  Note: this module only implements bare DXT encoding/decoding, it does not parse formats that can contain DXT files like .dds

    use std::io::{self, Read, Seek, SeekFrom, Write};

    use color::ColorType;
    use image::{self, ImageDecoder, ImageDecoderExt, ImageError, ImageReadBuffer, ImageResult, Progress};

    /// What version of DXT compression are we using?
    /// Note that DXT2 and DXT4 are left away as they're
    /// just DXT3 and DXT5 with premultiplied alpha
    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
    pub enum DXTVariant {
        /// The DXT1 format. 48 bytes of RGB data in a 4x4 pixel square is
        /// compressed into an 8 byte block of DXT1 data
        DXT1,
        /// The DXT3 format. 64 bytes of RGBA data in a 4x4 pixel square is
        /// compressed into a 16 byte block of DXT3 data
        DXT3,
        /// The DXT5 format. 64 bytes of RGBA data in a 4x4 pixel square is
        /// compressed into a 16 byte block of DXT5 data
        DXT5,
    }

    impl DXTVariant {
        /// Returns the amount of bytes of raw image data
        /// that is encoded in a single DXTn block
        fn decoded_bytes_per_block(self) -> usize {
            match self {
                DXTVariant::DXT1 => 48,
                DXTVariant::DXT3 | DXTVariant::DXT5 => 64,
            }
        }

        /// Returns the amount of bytes per block of encoded DXTn data
        fn encoded_bytes_per_block(self) -> usize {
            match self {
                DXTVariant::DXT1 => 8,
                DXTVariant::DXT3 | DXTVariant::DXT5 => 16,
            }
        }

        /// Returns the colortype that is stored in this DXT variant
        pub fn colortype(self) -> ColorType {
            match self {
                DXTVariant::DXT1 => ColorType::RGB(8),
                DXTVariant::DXT3 | DXTVariant::DXT5 => ColorType::RGBA(8),
            }
        }
    }

    /// DXT decoder
    pub struct DXTDecoder<R: Read> {
        inner: R,
        width_blocks: u32,
        height_blocks: u32,
        variant: DXTVariant,
        row: u32,
    }

    impl<R: Read> DXTDecoder<R> {
        /// Create a new DXT decoder that decodes from the stream ```r```.
        /// As DXT is often stored as raw buffers with the width/height
        /// somewhere else the width and height of the image need
        /// to be passed in ```width``` and ```height```, as well as the
        /// DXT variant in ```variant```.
        /// width and height are required to be powers of 2 and at least 4.
        /// otherwise an error will be returned
        pub fn new(
            r: R,
            width: u32,
            height: u32,
            variant: DXTVariant,
        ) -> Result<DXTDecoder<R>, ImageError> {
            if width % 4 != 0 || height % 4 != 0 {
                return Err(ImageError::DimensionError);
            }
            let width_blocks = width / 4;
            let height_blocks = height / 4;
            Ok(DXTDecoder {
                inner: r,
                width_blocks,
                height_blocks,
                variant,
                row: 0,
            })
        }

        fn read_scanline(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            assert_eq!(buf.len() as u64, self.scanline_bytes());

            let mut src =
                vec![0u8; self.variant.encoded_bytes_per_block() * self.width_blocks as usize];
            self.inner.read_exact(&mut src)?;
            match self.variant {
                DXTVariant::DXT1 => decode_dxt1_row(&src, buf),
                DXTVariant::DXT3 => decode_dxt3_row(&src, buf),
                DXTVariant::DXT5 => decode_dxt5_row(&src, buf),
            }
            self.row += 1;
            Ok(buf.len())
        }
    }

    // Note that, due to the way that DXT compression works, a scanline is considered to consist out of
    // 4 lines of pixels.
    impl<R: Read> ImageDecoder for DXTDecoder<R> {
        type Reader = DXTReader<R>;

        fn dimensions(&self) -> (u64, u64) {
            (self.width_blocks as u64 * 4, self.height_blocks as u64 * 4)
        }

        fn colortype(&self) -> ColorType {
            self.variant.colortype()
        }

        fn scanline_bytes(&self) -> u64 {
            self.variant.decoded_bytes_per_block() as u64 * self.width_blocks as u64
        }

        fn into_reader(self) -> ImageResult<Self::Reader> {
            if self.total_bytes() > usize::max_value() as u64 {
                return Err(ImageError::InsufficientMemory);
            }

            Ok(DXTReader {
                buffer: ImageReadBuffer::new(self.scanline_bytes() as usize, self.total_bytes() as usize),
                decoder: self,
            })
        }

        fn read_image(mut self) -> ImageResult<Vec<u8>> {
            if self.total_bytes() > usize::max_value() as u64 {
                return Err(ImageError::InsufficientMemory);
            }

            let mut dest = vec![0u8; self.total_bytes() as usize];
            for chunk in dest.chunks_mut(self.scanline_bytes() as usize) {
                self.read_scanline(chunk)?;
            }
            Ok(dest)
        }
    }

    impl<R: Read + Seek> ImageDecoderExt for DXTDecoder<R> {
        fn read_rect_with_progress<F: Fn(Progress)>(
            &mut self,
            x: u64,
            y: u64,
            width: u64,
            height: u64,
            buf: &mut [u8],
            progress_callback: F,
        ) -> ImageResult<()> {
            let encoded_scanline_bytes = self.variant.encoded_bytes_per_block() as u64
                * self.width_blocks as u64;

            let start = self.inner.seek(SeekFrom::Current(0))?;
            image::load_rect(x, y, width, height, buf, progress_callback, self,
                             |s, scanline| {
                                 s.inner.seek(SeekFrom::Start(start + scanline * encoded_scanline_bytes))?;
                                 Ok(())
                             },
                             |s, buf| s.read_scanline(buf))?;
            self.inner.seek(SeekFrom::Start(start))?;
            Ok(())
        }
    }

    /// DXT reader
    pub struct DXTReader<R: Read> {
        buffer: ImageReadBuffer,
        decoder: DXTDecoder<R>,
    }
    impl<R: Read> Read for DXTReader<R> {
        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            let ref mut decoder = &mut self.decoder;
            self.buffer.read(buf, |buf| decoder.read_scanline(buf))
        }
    }

    /// DXT encoder
    pub struct DXTEncoder<W: Write> {
        w: W,
    }

    impl<W: Write> DXTEncoder<W> {
        /// Create a new encoder that writes its output to ```w```
        pub fn new(w: W) -> DXTEncoder<W> {
            DXTEncoder { w }
        }

        /// Encodes the image data ```data```
        /// that has dimensions ```width``` and ```height```
        /// in ```DXTVariant``` ```variant```
        /// data is assumed to be in variant.colortype()
        pub fn encode(
            mut self,
            data: &[u8],
            width: u32,
            height: u32,
            variant: DXTVariant,
        ) -> ImageResult<()> {
            if width % 4 != 0 || height % 4 != 0 {
                return Err(ImageError::DimensionError);
            }
            let width_blocks = width / 4;
            let height_blocks = height / 4;

            let stride = variant.decoded_bytes_per_block();

            assert!(data.len() >= width_blocks as usize * height_blocks as usize * stride);

            for chunk in data.chunks(width_blocks as usize * stride) {
                let data = match variant {
                    DXTVariant::DXT1 => encode_dxt1_row(chunk),
                    DXTVariant::DXT3 => encode_dxt3_row(chunk),
                    DXTVariant::DXT5 => encode_dxt5_row(chunk),
                };
                self.w.write_all(&data)?;
            }
            Ok(())
        }
    }

    /**
     * Actual encoding/decoding logic below.
     */
    use std::mem::swap;

    type Rgb = [u8; 3];

    /// decodes a 5-bit R, 6-bit G, 5-bit B 16-bit packed color value into 8-bit RGB
    /// mapping is done so min/max range values are preserved. So for 5-bit
    /// values 0x00 -> 0x00 and 0x1F -> 0xFF
    fn enc565_decode(value: u16) -> Rgb {
        let red = (value >> 11) & 0x1F;
        let green = (value >> 5) & 0x3F;
        let blue = (value) & 0x1F;
        [
            (red * 0xFF / 0x1F) as u8,
            (green * 0xFF / 0x3F) as u8,
            (blue * 0xFF / 0x1F) as u8,
        ]
    }

    /// encodes an 8-bit RGB value into a 5-bit R, 6-bit G, 5-bit B 16-bit packed color value
    /// mapping preserves min/max values. It is guaranteed that i == encode(decode(i)) for all i
    fn enc565_encode(rgb: Rgb) -> u16 {
        let red = (u16::from(rgb[0]) * 0x1F + 0x7E) / 0xFF;
        let green = (u16::from(rgb[1]) * 0x3F + 0x7E) / 0xFF;
        let blue = (u16::from(rgb[2]) * 0x1F + 0x7E) / 0xFF;
        (red << 11) | (green << 5) | blue
    }

    /// utility function: squares a value
    fn square(a: i32) -> i32 {
        a * a
    }

    /// returns the squared error between two RGB values
    fn diff(a: Rgb, b: Rgb) -> i32 {
        square(i32::from(a[0]) - i32::from(b[0])) + square(i32::from(a[1]) - i32::from(b[1]))
            + square(i32::from(a[2]) - i32::from(b[2]))
    }

    /*
     * Functions for decoding DXT compression
     */

    /// Constructs the DXT5 alpha lookup table from the two alpha entries
    /// if alpha0 > alpha1, constructs a table of [a0, a1, 6 linearly interpolated values from a0 to a1]
    /// if alpha0 <= alpha1, constructs a table of [a0, a1, 4 linearly interpolated values from a0 to a1, 0, 0xFF]
    fn alpha_table_dxt5(alpha0: u8, alpha1: u8) -> [u8; 8] {
        let mut table = [alpha0, alpha1, 0, 0, 0, 0, 0, 0xFF];
        if alpha0 > alpha1 {
            for i in 2..8u16 {
                table[i as usize] =
                    (((8 - i) * u16::from(alpha0) + (i - 1) * u16::from(alpha1)) / 7) as u8;
            }
        } else {
            for i in 2..6u16 {
                table[i as usize] =
                    (((6 - i) * u16::from(alpha0) + (i - 1) * u16::from(alpha1)) / 5) as u8;
            }
        }
        table
    }

    /// decodes an 8-byte dxt color block into the RGB channels of a 16xRGB or 16xRGBA block.
    /// source should have a length of 8, dest a length of 48 (RGB) or 64 (RGBA)
    fn decode_dxt_colors(source: &[u8], dest: &mut [u8]) {
        // sanity checks, also enable the compiler to elide all following bound checks
        assert!(source.len() == 8 && (dest.len() == 48 || dest.len() == 64));
        // calculate pitch to store RGB values in dest (3 for RGB, 4 for RGBA)
        let pitch = dest.len() / 16;

        // extract color data
        let color0 = u16::from(source[0]) | (u16::from(source[1]) << 8);
        let color1 = u16::from(source[2]) | (u16::from(source[3]) << 8);
        let color_table = u32::from(source[4]) | (u32::from(source[5]) << 8)
            | (u32::from(source[6]) << 16) | (u32::from(source[7]) << 24);
        // let color_table = source[4..8].iter().rev().fold(0, |t, &b| (t << 8) | b as u32);

        // decode the colors to rgb format
        let mut colors = [[0; 3]; 4];
        colors[0] = enc565_decode(color0);
        colors[1] = enc565_decode(color1);

        // determine color interpolation method
        if color0 > color1 {
            // linearly interpolate the other two color table entries
            for i in 0..3 {
                colors[2][i] = ((u16::from(colors[0][i]) * 2 + u16::from(colors[1][i]) + 1) / 3) as u8;
                colors[3][i] = ((u16::from(colors[0][i]) + u16::from(colors[1][i]) * 2 + 1) / 3) as u8;
            }
        } else {
            // linearly interpolate one other entry, keep the other at 0
            for i in 0..3 {
                colors[2][i] = ((u16::from(colors[0][i]) + u16::from(colors[1][i]) + 1) / 2) as u8;
            }
        }

        // serialize the result. Every color is determined by looking up
        // two bits in color_table which identify which color to actually pick from the 4 possible colors
        for i in 0..16 {
            dest[i * pitch..i * pitch + 3]
                .copy_from_slice(&colors[(color_table >> (i * 2)) as usize & 3]);
        }
    }

    /// Decodes a 16-byte bock of dxt5 data to a 16xRGBA block
    fn decode_dxt5_block(source: &[u8], dest: &mut [u8]) {
        assert!(source.len() == 16 && dest.len() == 64);

        // extract alpha index table (stored as little endian 64-bit value)
        let alpha_table = source[2..8]
            .iter()
            .rev()
            .fold(0, |t, &b| (t << 8) | u64::from(b));

        // alhpa level decode
        let alphas = alpha_table_dxt5(source[0], source[1]);

        // serialize alpha
        for i in 0..16 {
            dest[i * 4 + 3] = alphas[(alpha_table >> (i * 3)) as usize & 7];
        }

        // handle colors
        decode_dxt_colors(&source[8..16], dest);
    }

    /// Decodes a 16-byte bock of dxt3 data to a 16xRGBA block
    fn decode_dxt3_block(source: &[u8], dest: &mut [u8]) {
        assert!(source.len() == 16 && dest.len() == 64);

        // extract alpha index table (stored as little endian 64-bit value)
        let alpha_table = source[0..8]
            .iter()
            .rev()
            .fold(0, |t, &b| (t << 8) | u64::from(b));

        // serialize alpha (stored as 4-bit values)
        for i in 0..16 {
            dest[i * 4 + 3] = ((alpha_table >> (i * 4)) as u8 & 0xF) * 0x11;
        }

        // handle colors
        decode_dxt_colors(&source[8..16], dest);
    }

    /// Decodes a 8-byte bock of dxt5 data to a 16xRGB block
    fn decode_dxt1_block(source: &[u8], dest: &mut [u8]) {
        assert!(source.len() == 8 && dest.len() == 48);
        decode_dxt_colors(&source, dest);
    }

    /// Decode a row of DXT1 data to four rows of RGBA data.
    /// source.len() should be a multiple of 8, otherwise this panics.
    fn decode_dxt1_row(source: &[u8], dest: &mut [u8]) {
        assert!(source.len() % 8 == 0);
        let block_count = source.len() / 8;
        assert!(dest.len() >= block_count * 48);

        // contains the 16 decoded pixels per block
        let mut decoded_block = [0u8; 48];

        for (x, encoded_block) in source.chunks(8).enumerate() {
            decode_dxt1_block(encoded_block, &mut decoded_block);

            // copy the values from the decoded block to linewise RGB layout
            for line in 0..4 {
                let offset = (block_count * line + x) * 12;
                dest[offset..offset + 12].copy_from_slice(&decoded_block[line * 12..(line + 1) * 12]);
            }
        }
    }

    /// Decode a row of DXT3 data to four rows of RGBA data.
    /// source.len() should be a multiple of 16, otherwise this panics.
    fn decode_dxt3_row(source: &[u8], dest: &mut [u8]) {
        assert!(source.len() % 16 == 0);
        let block_count = source.len() / 16;
        assert!(dest.len() >= block_count * 64);

        // contains the 16 decoded pixels per block
        let mut decoded_block = [0u8; 64];

        for (x, encoded_block) in source.chunks(16).enumerate() {
            decode_dxt3_block(encoded_block, &mut decoded_block);

            // copy the values from the decoded block to linewise RGB layout
            for line in 0..4 {
                let offset = (block_count * line + x) * 16;
                dest[offset..offset + 16].copy_from_slice(&decoded_block[line * 16..(line + 1) * 16]);
            }
        }
    }

    /// Decode a row of DXT5 data to four rows of RGBA data.
    /// source.len() should be a multiple of 16, otherwise this panics.
    fn decode_dxt5_row(source: &[u8], dest: &mut [u8]) {
        assert!(source.len() % 16 == 0);
        let block_count = source.len() / 16;
        assert!(dest.len() >= block_count * 64);

        // contains the 16 decoded pixels per block
        let mut decoded_block = [0u8; 64];

        for (x, encoded_block) in source.chunks(16).enumerate() {
            decode_dxt5_block(encoded_block, &mut decoded_block);

            // copy the values from the decoded block to linewise RGB layout
            for line in 0..4 {
                let offset = (block_count * line + x) * 16;
                dest[offset..offset + 16].copy_from_slice(&decoded_block[line * 16..(line + 1) * 16]);
            }
        }
    }

    /*
     * Functions for encoding DXT compression
     */

    /// Tries to perform the color encoding part of dxt compression
    /// the approach taken is simple, it picks unique combinations
    /// of the colors present in the block, and attempts to encode the
    /// block with each, picking the encoding that yields the least
    /// squared error out of all of them.
    ///
    /// This could probably be faster but is already reasonably fast
    /// and a good reference impl to optimize others against.
    ///
    /// Another way to perform this analysis would be to perform a
    /// singular value decomposition of the different colors, and
    /// then pick 2 points on this line as the base colors. But
    /// this is still rather unwieldly math and has issues
    /// with the 3-linear-colors-and-0 case, it's also worse
    /// at conserving the original colors.
    ///
    /// source: should be RGBAx16 or RGBx16 bytes of data,
    /// dest 8 bytes of resulting encoded color data
    fn encode_dxt_colors(source: &[u8], dest: &mut [u8]) {
        // sanity checks and determine stride when parsing the source data
        assert!((source.len() == 64 || source.len() == 48) && dest.len() == 8);
        let stride = source.len() / 16;

        // reference colors array
        let mut colors = [[0u8; 3]; 4];

        // Put the colors we're going to be processing in an array with pure RGB layout
        // note: we reverse the pixel order here. The reason for this is found in the inner quantization loop.
        let mut targets = [[0u8; 3]; 16];
        for (s, d) in source.chunks(stride).rev().zip(&mut targets) {
            *d = [s[0], s[1], s[2]];
        }

        // and a set of colors to pick from.
        let mut colorspace = targets.to_vec();

        // roundtrip all colors through the r5g6b5 encoding
        for rgb in &mut colorspace {
            *rgb = enc565_decode(enc565_encode(*rgb));
        }

        // and deduplicate the set of colors to choose from as the algorithm is O(N^2) in this
        colorspace.dedup();

        // in case of slight gradients it can happen that there's only one entry left in the color table.
        // as the resulting banding can be quite bad if we would just left the block at the closest
        // encodable color, we have a special path here that tries to emulate the wanted color
        // using the linear interpolation between gradients
        if colorspace.len() == 1 {
            // the base color we got from colorspace reduction
            let ref_rgb = colorspace[0];
            // the unreduced color in this block that's the furthest away from the actual block
            let mut rgb = targets
                .iter()
                .cloned()
                .max_by_key(|rgb| diff(*rgb, ref_rgb))
                .unwrap();
            // amplify differences by 2.5, which should push them to the next quantized value
            // if possible without overshoot
            for i in 0..3 {
                rgb[i] =
                    ((i16::from(rgb[i]) - i16::from(ref_rgb[i])) * 5 / 2 + i16::from(ref_rgb[i])) as u8;
            }

            // roundtrip it through quantization
            let encoded = enc565_encode(rgb);
            let rgb = enc565_decode(encoded);

            // in case this didn't land us a different color the best way to represent this field is
            // as a single color block
            if rgb == ref_rgb {
                dest[0] = encoded as u8;
                dest[1] = (encoded >> 8) as u8;

                for d in dest.iter_mut().take(8).skip(2) {
                    *d = 0;
                }
                return;
            }

            // we did find a separate value: add it to the options so after one round of quantization
            // we're done
            colorspace.push(rgb);
        }

        // block quantization loop: we basically just try every possible combination, returning
        // the combination with the least squared error
        // stores the best candidate colors
        let mut chosen_colors = [[0; 3]; 4];
        // did this index table use the [0,0,0] variant
        let mut chosen_use_0 = false;
        // error calculated for the last entry
        let mut chosen_error = 0xFFFF_FFFFu32;

        // loop through unique permutations of the colorspace, where c1 != c2
        'search: for (i, &c1) in colorspace.iter().enumerate() {
            colors[0] = c1;

            for &c2 in &colorspace[0..i] {
                colors[1] = c2;

                // what's inside here is ran at most 120 times.
                for use_0 in 0..2 {
                    // and 240 times here.

                    if use_0 != 0 {
                        // interpolate one color, set the other to 0
                        for i in 0..3 {
                            colors[2][i] =
                                ((u16::from(colors[0][i]) + u16::from(colors[1][i]) + 1) / 2) as u8;
                        }
                        colors[3] = [0, 0, 0];
                    } else {
                        // interpolate to get 2 more colors
                        for i in 0..3 {
                            colors[2][i] =
                                ((u16::from(colors[0][i]) * 2 + u16::from(colors[1][i]) + 1) / 3) as u8;
                            colors[3][i] =
                                ((u16::from(colors[0][i]) + u16::from(colors[1][i]) * 2 + 1) / 3) as u8;
                        }
                    }

                    // calculate the total error if we were to quantize the block with these color combinations
                    // both these loops have statically known iteration counts and are well vectorizable
                    // note that the inside of this can be run about 15360 times worst case, i.e. 960 times per
                    // pixel.
                    let total_error = targets
                        .iter()
                        .map(|t| colors.iter().map(|c| diff(*c, *t) as u32).min().unwrap())
                        .sum();

                    // update the match if we found a better one
                    if total_error < chosen_error {
                        chosen_colors = colors;
                        chosen_use_0 = use_0 != 0;
                        chosen_error = total_error;

                        // if we've got a perfect or at most 1 LSB off match, we're done
                        if total_error < 4 {
                            break 'search;
                        }
                    }
                }
            }
        }

        // calculate the final indices
        // note that targets is already in reverse pixel order, to make the index computation easy.
        let mut chosen_indices = 0u32;
        for t in &targets {
            let (idx, _) = chosen_colors
                .iter()
                .enumerate()
                .min_by_key(|&(_, c)| diff(*c, *t))
                .unwrap();
            chosen_indices = (chosen_indices << 2) | idx as u32;
        }

        // encode the colors
        let mut color0 = enc565_encode(chosen_colors[0]);
        let mut color1 = enc565_encode(chosen_colors[1]);

        // determine encoding. Note that color0 == color1 is impossible at this point
        if color0 > color1 {
            if chosen_use_0 {
                swap(&mut color0, &mut color1);
                // Indexes are packed 2 bits wide, swap index 0/1 but preserve 2/3.
                let filter = (chosen_indices & 0xAAAA_AAAA) >> 1;
                chosen_indices ^= filter ^ 0x5555_5555;
            }
        } else if !chosen_use_0 {
            swap(&mut color0, &mut color1);
            // Indexes are packed 2 bits wide, swap index 0/1 and 2/3.
            chosen_indices ^= 0x5555_5555;
        }

        // encode everything.
        dest[0] = color0 as u8;
        dest[1] = (color0 >> 8) as u8;
        dest[2] = color1 as u8;
        dest[3] = (color1 >> 8) as u8;
        for i in 0..4 {
            dest[i + 4] = (chosen_indices >> (i * 8)) as u8;
        }
    }

    /// Encodes a buffer of 16 alpha bytes into a dxt5 alpha index table,
    /// where the alpha table they are indexed against is created by
    /// calling alpha_table_dxt5(alpha0, alpha1)
    /// returns the resulting error and alpha table
    fn encode_dxt5_alpha(alpha0: u8, alpha1: u8, alphas: &[u8; 16]) -> (i32, u64) {
        // create a table for the given alpha ranges
        let table = alpha_table_dxt5(alpha0, alpha1);
        let mut indices = 0u64;
        let mut total_error = 0i32;

        // least error brute force search
        for (i, &a) in alphas.iter().enumerate() {
            let (index, error) = table
                .iter()
                .enumerate()
                .map(|(i, &e)| (i, square(i32::from(e) - i32::from(a))))
                .min_by_key(|&(_, e)| e)
                .unwrap();
            total_error += error;
            indices |= (index as u64) << (i * 3);
        }

        (total_error, indices)
    }

    /// Encodes a RGBAx16 sequence of bytes to a 16 bytes DXT5 block
    fn encode_dxt5_block(source: &[u8], dest: &mut [u8]) {
        assert!(source.len() == 64 && dest.len() == 16);

        // perform dxt color encoding
        encode_dxt_colors(source, &mut dest[8..16]);

        // copy out the alpha bytes
        let mut alphas = [0; 16];
        for i in 0..16 {
            alphas[i] = source[i * 4 + 3];
        }

        // try both alpha compression methods, see which has the least error.
        let alpha07 = alphas.iter().cloned().min().unwrap();
        let alpha17 = alphas.iter().cloned().max().unwrap();
        let (error7, indices7) = encode_dxt5_alpha(alpha07, alpha17, &alphas);

        // if all alphas are 0 or 255 it doesn't particularly matter what we do here.
        let alpha05 = alphas
            .iter()
            .cloned()
            .filter(|&i| i != 255)
            .max()
            .unwrap_or(255);
        let alpha15 = alphas
            .iter()
            .cloned()
            .filter(|&i| i != 0)
            .min()
            .unwrap_or(0);
        let (error5, indices5) = encode_dxt5_alpha(alpha05, alpha15, &alphas);

        // pick the best one, encode the min/max values
        let mut alpha_table = if error5 < error7 {
            dest[0] = alpha05;
            dest[1] = alpha15;
            indices5
        } else {
            dest[0] = alpha07;
            dest[1] = alpha17;
            indices7
        };

        // encode the alphas
        for byte in dest[2..8].iter_mut() {
            *byte = alpha_table as u8;
            alpha_table >>= 8;
        }
    }

    /// Encodes a RGBAx16 sequence of bytes into a 16 bytes DXT3 block
    fn encode_dxt3_block(source: &[u8], dest: &mut [u8]) {
        assert!(source.len() == 64 && dest.len() == 16);

        // perform dxt color encoding
        encode_dxt_colors(source, &mut dest[8..16]);

        // DXT3 alpha compression is very simple, just round towards the nearest value

        // index the alpha values into the 64bit alpha table
        let mut alpha_table = 0u64;
        for i in 0..16 {
            let alpha = u64::from(source[i * 4 + 3]);
            let alpha = (alpha + 0x8) / 0x11;
            alpha_table |= alpha << (i * 4);
        }

        // encode the alpha values
        for byte in &mut dest[0..8] {
            *byte = alpha_table as u8;
            alpha_table >>= 8;
        }
    }

    /// Encodes a RGBx16 sequence of bytes into a 8 bytes DXT1 block
    fn encode_dxt1_block(source: &[u8], dest: &mut [u8]) {
        assert!(source.len() == 48 && dest.len() == 8);

        // perform dxt color encoding
        encode_dxt_colors(source, dest);
    }

    /// Decode a row of DXT1 data to four rows of RGBA data.
    /// source.len() should be a multiple of 8, otherwise this panics.
    fn encode_dxt1_row(source: &[u8]) -> Vec<u8> {
        assert!(source.len() % 48 == 0);
        let block_count = source.len() / 48;

        let mut dest = vec![0u8; block_count * 8];
        // contains the 16 decoded pixels per block
        let mut decoded_block = [0u8; 48];

        for (x, encoded_block) in dest.chunks_mut(8).enumerate() {
            // copy the values from the decoded block to linewise RGB layout
            for line in 0..4 {
                let offset = (block_count * line + x) * 12;
                decoded_block[line * 12..(line + 1) * 12].copy_from_slice(&source[offset..offset + 12]);
            }

            encode_dxt1_block(&decoded_block, encoded_block);
        }
        dest
    }

    /// Decode a row of DXT3 data to four rows of RGBA data.
    /// source.len() should be a multiple of 16, otherwise this panics.
    fn encode_dxt3_row(source: &[u8]) -> Vec<u8> {
        assert!(source.len() % 64 == 0);
        let block_count = source.len() / 64;

        let mut dest = vec![0u8; block_count * 16];
        // contains the 16 decoded pixels per block
        let mut decoded_block = [0u8; 64];

        for (x, encoded_block) in dest.chunks_mut(16).enumerate() {
            // copy the values from the decoded block to linewise RGB layout
            for line in 0..4 {
                let offset = (block_count * line + x) * 16;
                decoded_block[line * 16..(line + 1) * 16].copy_from_slice(&source[offset..offset + 16]);
            }

            encode_dxt3_block(&decoded_block, encoded_block);
        }
        dest
    }

    /// Decode a row of DXT5 data to four rows of RGBA data.
    /// source.len() should be a multiple of 16, otherwise this panics.
    fn encode_dxt5_row(source: &[u8]) -> Vec<u8> {
        assert!(source.len() % 64 == 0);
        let block_count = source.len() / 64;

        let mut dest = vec![0u8; block_count * 16];
        // contains the 16 decoded pixels per block
        let mut decoded_block = [0u8; 64];

        for (x, encoded_block) in dest.chunks_mut(16).enumerate() {
            // copy the values from the decoded block to linewise RGB layout
            for line in 0..4 {
                let offset = (block_count * line + x) * 16;
                decoded_block[line * 16..(line + 1) * 16].copy_from_slice(&source[offset..offset + 16]);
            }

            encode_dxt5_block(&decoded_block, encoded_block);
        }
        dest
    }

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)