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// PNG 元数据读取（纯 MoonBit）：text chunks (tEXt/iTXt)
///|
///|
/// PNG text chunk（关键字-文本对）
pub(all) struct PngTextChunk {
keyword : String
text : String
} derive(Eq, @debug.Debug)
///|
fn read_png_bytes_as_string(data : Bytes, start : Int, end : Int) -> String {
let chars : Array[Byte] = []
for i in start..<end {
chars.push(data[i])
}
@encoding/utf8.decode(Bytes::from_array(chars)[:]) catch {
_ => ""
}
}
///|
/// 从 PNG 字节流读取 text chunks（tEXt 和 iTXt）。
/// 返回关键字-文本对数组。不含 EXIF 等其他元数据。
pub fn read_png_text_chunks(data : Bytes) -> Array[PngTextChunk] {
let result : Array[PngTextChunk] = []
let len = data.length()
// PNG signature: 8 bytes
if len < 8 ||
data[0].to_int() != 0x89 ||
data[1].to_int() != 0x50 ||
data[2].to_int() != 0x4E ||
data[3].to_int() != 0x47 {
return result
}
let mut pos = 8
while pos + 8 <= len {
// chunk length (4 bytes BE) + type (4 bytes)
let chunk_len = data[pos].to_int() * 16777216 +
data[pos + 1].to_int() * 65536 +
data[pos + 2].to_int() * 256 +
data[pos + 3].to_int()
let type0 = data[pos + 4].to_int()
let type1 = data[pos + 5].to_int()
let type2 = data[pos + 6].to_int()
let type3 = data[pos + 7].to_int()
let data_start = pos + 8
if data_start + chunk_len > len {
break
}
// tEXt: type = 0x74 0x45 0x58 0x74
if type0 == 0x74 && type1 == 0x45 && type2 == 0x58 && type3 == 0x74 {
// tEXt: keyword + 0x00 + text
let mut sep = -1
for i in 0..<chunk_len {
if data[data_start + i] == b'\x00' {
sep = i
break
}
}
if sep >= 0 {
let keyword = read_png_bytes_as_string(
data,
data_start,
data_start + sep,
)
let text = read_png_bytes_as_string(
data,
data_start + sep + 1,
data_start + chunk_len,
)
result.push(PngTextChunk::{ keyword, text })
}
}
// iTXt: type = 0x69 0x54 0x58 0x74
if type0 == 0x69 && type1 == 0x54 && type2 == 0x58 && type3 == 0x74 {
// iTXt: keyword + 0x00 + compression_flag + compression_method + language_tag + 0x00 + translated_keyword + 0x00 + text
let mut sep = -1
for i in 0..<chunk_len {
if data[data_start + i] == b'\x00' {
sep = i
break
}
}
if sep >= 0 && sep + 2 < chunk_len {
let keyword = read_png_bytes_as_string(
data,
data_start,
data_start + sep,
)
let comp_flag = data[data_start + sep + 1].to_int()
// 跳过 compression_method (1 byte) + language_tag + 0x00 + translated_keyword + 0x00
let mut text_start = sep + 3 // skip comp_flag + comp_method
// 跳过 language tag (until 0x00)
while text_start < chunk_len && data[data_start + text_start] != b'\x00' {
text_start += 1
}
text_start += 1 // skip 0x00
// 跳过 translated keyword (until 0x00)
while text_start < chunk_len && data[data_start + text_start] != b'\x00' {
text_start += 1
}
text_start += 1 // skip 0x00
if text_start <= chunk_len {
let text = if comp_flag == 0 {
read_png_bytes_as_string(
data,
data_start + text_start,
data_start + chunk_len,
)
} else {
// 压缩文本不支持（需要 zlib 解压）
"[compressed]"
}
result.push(PngTextChunk::{ keyword, text })
}
}
}
// IEND 标志着 PNG 结束
if type0 == 0x49 && type1 == 0x45 && type2 == 0x4E && type3 == 0x44 {
break
}
// 前进到下一个 chunk: length(4) + type(4) + data(chunk_len) + CRC(4)
pos = data_start + chunk_len + 4
}
result
}