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

use crate::types::{ColorMap, ExtensionBlock, Frame, Gif, GraphicControlExtension};
use std::io::Read;
#[cfg(test)]
use {std::fs::File, std::path::Path};

mod steps;

/// Color output mode.
/// This setting affects the raster data of each decoded gif frame.
#[derive(PartialEq)]
pub enum ColorOutput {
    /// Every byte of the raster data is expanded to 4 bytes (R G B A).
    /// In this mode the ColorMap is useless, for this reason it is not returned in the Gif object.
    RGBA,
    /// Normal ColorMap index color mapping.
    ColorMap,
}

/// Decode a gif encoded source.
pub fn decode(
    mut source: impl Read,
    color_output: ColorOutput,
) -> Result<Gif, Box<dyn std::error::Error>> {
    let bytes: &mut Vec<u8> = &mut Vec::new();
    source.read_to_end(bytes)?;

    let (signature, cursor) = steps::signature::decode(&bytes, 0);
    let (mut screen_descriptor, cursor) = steps::screen_descriptor::decode(&bytes, cursor);
    let (mut global_color_map, cursor) =
        steps::color_map::decode(&bytes, screen_descriptor.pixel, screen_descriptor.m, cursor);
    let (mut frames, _cursor) = frames(bytes, cursor);

    if color_output == ColorOutput::RGBA {
        for frame in &mut frames {
            let rgba_raster_data = rgba_raster_data(&frame, global_color_map.as_ref());
            frame.raster_data = rgba_raster_data;
            frame.local_color_map = None;
            frame.image_descriptor.m = false;
        }
        screen_descriptor.m = false;
        global_color_map = None;
    }

    Ok(Gif {
        signature: signature.to_string(),
        screen_descriptor,
        global_color_map,
        frames,
    })
}

fn rgba_raster_data(frame: &Frame, global_color_map: Option<&ColorMap>) -> Vec<u8> {
    let color_map = if frame.image_descriptor.m {
        frame
            .local_color_map
            .as_ref()
            .expect("expected local color map not present")
    } else {
        global_color_map.expect("expected global color map not present")
    };

    frame
        .raster_data
        .iter()
        .map(|index| {
            table_index_to_rgba(
                *index,
                color_map,
                frame
                    .graphic_control_extension
                    .as_ref()
                    .and_then(|ext| ext.transparent_color_index),
            )
        })
        .flatten()
        .collect()
}

fn table_index_to_rgba(
    index: u8,
    color_map: &ColorMap,
    maybe_transparent_color_index: Option<u8>,
) -> Vec<u8> {
    let rgba = color_map
        .get(&(index as usize))
        .expect("pixel index not found in color map");
    let alpha = maybe_transparent_color_index
        .filter(|alpha_index| index == *alpha_index)
        .map(|_| 0x00u8)
        .unwrap_or(0xFFu8);
    vec![rgba.r, rgba.g, rgba.b, alpha]
}

fn frames(bytes: &Vec<u8>, cursor: usize) -> (Vec<Frame>, usize) {
    let mut mut_index = cursor;
    let mut frames: Vec<Frame> = Vec::new();

    while bytes[mut_index] != 0x3b {
        let (frame, index) = frame(bytes, mut_index);
        mut_index = index;
        frames.push(frame);
    }

    (frames, mut_index)
}

fn frame(bytes: &Vec<u8>, cursor: usize) -> (Frame, usize) {
    let mut index = cursor;
    let mut graphic_control_extension: Option<GraphicControlExtension> = None;

    while bytes[index] != 0x2c {
        if bytes[index] == 0x21 {
            let (block, cursor) = steps::extension_block::decode(bytes, index);
            index = cursor;

            match block {
                Some(ExtensionBlock::GraphicControlExtension(extension)) => {
                    graphic_control_extension = Some(extension);
                }
                _ => {}
            }
        } else {
            index += 1;
        }
    }

    let (image_descriptor, index) = steps::image_descriptor::decode(bytes, index);
    let (color_map, index) =
        steps::color_map::decode(bytes, image_descriptor.pixel, image_descriptor.m, index);
    let (raster_data, index) = steps::raster_data::decode(bytes, index);

    (
        Frame {
            image_descriptor,
            local_color_map: color_map,
            raster_data,
            graphic_control_extension,
        },
        index,
    )
}

#[test]
pub fn should_decode() {
    let file = &mut File::open(Path::new("./ascii-gif-example.gif")).unwrap();
    let gif = decode(file, ColorOutput::ColorMap).unwrap();

    assert_eq!("GIF89a", gif.signature);
    assert_eq!(106, gif.frames.len());
    gif.frames.iter().for_each(|frame| {
        assert_eq!(
            frame.raster_data.len(),
            (frame.image_descriptor.image_width as u32 * frame.image_descriptor.image_height as u32)
                as usize
        );

        if frame.image_descriptor.m {
            assert!(frame.local_color_map.is_some())
        } else {
            assert!(frame.local_color_map.is_none())
        }
    });
}