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// 纯 MoonBit PNG 解码（全目标可用：native/wasm/js）
// 依赖 @pure/zlib 进行 IDAT 解压缩
// 支持 color type 0/2/3/4/6, bit depth 8/16
///|
/// PNG 颜色类型对应的通道数
fn png_channels(color_type : Int) -> Int? {
match color_type {
0 => Some(1) // gray
2 => Some(3) // rgb
3 => Some(1) // palette
4 => Some(2) // gray + alpha
6 => Some(4) // rgba
_ => None
}
}
///|
/// Paeth 预测函数
fn paeth_predict(a : Int, b : Int, c : Int) -> Int {
let p = a + b - c
let pa = (p - a).abs()
let pb = (p - b).abs()
let pc = (p - c).abs()
if pa <= pb && pa <= pc {
a
} else if pb <= pc {
b
} else {
c
}
}
///|
/// PNG 解码（纯 MoonBit，全目标可用）。
/// 支持 8-bit gray/RGB/RGBA/gray+alpha/palette。
/// 不支持 interlace。
/// req_channels: None=原色（保留 alpha）, 1=灰度, 2=灰度+alpha, 3=RGB, 4=RGBA
pub fn decode_png_pure(
data : Bytes,
req_channels? : Int? = None,
) -> @types.Image raise @types.LoadError {
let len = data.length()
// 验证 PNG signature
if len < 8 || !is_png_signature(data, 0) {
raise @types.LoadError::DecodeFailed("PNG signature 错误")
}
let mut pos = 8
// 解析 chunks
let mut width = 0
let mut height = 0
let mut bit_depth = 0
let mut color_type = 0
let mut interlace = 0
let palette : Array[(Int, Int, Int)] = []
let idat_data : Array[Byte] = []
let mut found_ihdr = false
let mut found_iend = false
while pos < len && !found_iend {
if pos + 8 > len {
raise @types.LoadError::DecodeFailed("PNG chunk 头部过短")
}
let chunk_len = read_u32_be(data, pos)
if chunk_len < 0 {
raise @types.LoadError::DecodeFailed("PNG chunk 长度无效")
}
let chunk_key = read_u32_be(data, pos + 4)
pos += 8
if pos + chunk_len > len {
raise @types.LoadError::DecodeFailed("PNG chunk 数据越界")
}
match chunk_key {
0x49484452 => {
// IHDR
if chunk_len < 13 {
raise @types.LoadError::DecodeFailed("PNG IHDR 过短")
}
width = read_u32_be(data, pos)
height = read_u32_be(data, pos + 4)
bit_depth = data[pos + 8].to_int()
color_type = data[pos + 9].to_int()
interlace = data[pos + 12].to_int()
found_ihdr = true
}
0x504C5445 => {
// PLTE
let num_colors = chunk_len / 3
for i in 0..<num_colors {
palette.push(
(
data[pos + i * 3].to_int(),
data[pos + i * 3 + 1].to_int(),
data[pos + i * 3 + 2].to_int(),
),
)
}
}
0x49444154 =>
// IDAT
for i in 0..<chunk_len {
idat_data.push(data[pos + i])
}
0x49454E44 =>
// IEND
found_iend = true
_ => ()
}
pos += chunk_len + 4 // skip data + CRC
}
if !found_ihdr {
raise @types.LoadError::DecodeFailed("PNG 无 IHDR chunk")
}
if width <= 0 || height <= 0 {
raise @types.LoadError::DecodeFailed("PNG 尺寸无效")
}
if interlace != 0 {
raise @types.LoadError::DecodeFailed("PNG interlace 不支持")
}
if bit_depth != 8 {
raise @types.LoadError::DecodeFailed("PNG 仅支持 8-bit bit depth")
}
let ch = match png_channels(color_type) {
Some(c) => c
None => raise @types.LoadError::DecodeFailed("PNG 无效 color type")
}
@types.check_dims(width, height, ch)
// zlib inflate IDAT 数据
let raw = @util.zlib_inflate(Bytes::from_array(idat_data))
// unfilter scanlines
let bpp = ch // bytes per pixel (8-bit)
let stride = width * bpp
let unfiltered : Array[Byte] = Array::make(height * stride, b'\x00')
let mut raw_pos = 0
for y in 0..<height {
if raw_pos >= raw.length() {
raise @types.LoadError::DecodeFailed("PNG 解压数据过短")
}
let filter_type = raw[raw_pos].to_int()
raw_pos += 1
for x in 0..<stride {
if raw_pos >= raw.length() {
raise @types.LoadError::DecodeFailed("PNG 解压数据过短")
}
let raw_byte = raw[raw_pos].to_int()
raw_pos += 1
let a = if x >= bpp {
unfiltered[y * stride + x - bpp].to_int()
} else {
0
}
let b = if y > 0 { unfiltered[(y - 1) * stride + x].to_int() } else { 0 }
let c = if x >= bpp && y > 0 {
unfiltered[(y - 1) * stride + x - bpp].to_int()
} else {
0
}
let val = match filter_type {
0 => raw_byte // None
1 => (raw_byte + a) & 0xFF // Sub
2 => (raw_byte + b) & 0xFF // Up
3 => (raw_byte + (a + b) / 2) & 0xFF // Average
4 => (raw_byte + paeth_predict(a, b, c)) & 0xFF // Paeth
_ => raise @types.LoadError::DecodeFailed("PNG 无效 filter type")
}
unfiltered[y * stride + x] = val.to_byte()
}
}
// 转换为 Image（保留 alpha，根据 req_channels 决定输出通道数）
let pixel_count = width * height
let out_ch = match req_channels {
Some(n) => n
None => ch // 默认保留原色（含 alpha）
}
if color_type == 3 && palette.length() == 0 {
raise @types.LoadError::DecodeFailed("PNG palette 缺失")
}
let out_data : Array[Byte] = Array::make(pixel_count * out_ch, b'\x00')
for i in 0..<pixel_count {
let (r, g, b, a) = match color_type {
3 => {
// palette
let idx = unfiltered[i].to_int()
if idx >= palette.length() {
raise @types.LoadError::DecodeFailed("PNG palette 索引越界")
}
let (r, g, b) = palette[idx]
(r, g, b, 255)
}
0 => {
// gray
let v = unfiltered[i].to_int()
(v, v, v, 255)
}
2 =>
// RGB
(
unfiltered[i * 3].to_int(),
unfiltered[i * 3 + 1].to_int(),
unfiltered[i * 3 + 2].to_int(),
255,
)
4 => {
// gray + alpha
let v = unfiltered[i * 2].to_int()
let a = unfiltered[i * 2 + 1].to_int()
(v, v, v, a)
}
_ =>
// RGBA
(
unfiltered[i * 4].to_int(),
unfiltered[i * 4 + 1].to_int(),
unfiltered[i * 4 + 2].to_int(),
unfiltered[i * 4 + 3].to_int(),
)
}
let (or, og, ob, oa) = match out_ch {
1 => {
let gray = (r * 299 + g * 587 + b * 114) / 1000
(gray, gray, gray, 255)
}
2 => {
let gray = (r * 299 + g * 587 + b * 114) / 1000
(gray, gray, gray, a)
}
3 => (r, g, b, 255)
_ => (r, g, b, a)
}
match out_ch {
1 => out_data[i] = or.to_byte()
2 => {
out_data[i * 2] = or.to_byte()
out_data[i * 2 + 1] = oa.to_byte()
}
3 => {
out_data[i * 3] = or.to_byte()
out_data[i * 3 + 1] = og.to_byte()
out_data[i * 3 + 2] = ob.to_byte()
}
_ => {
out_data[i * 4] = or.to_byte()
out_data[i * 4 + 1] = og.to_byte()
out_data[i * 4 + 2] = ob.to_byte()
out_data[i * 4 + 3] = oa.to_byte()
}
}
}
@types.Image::{
width,
height,
channels: out_ch,
data: Bytes::from_array(out_data),
}
}