`nx_bmp_header.nx` source

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1// bmp_header.nx -- parse Windows BMP/DIB image headers.
2//
3// BMP is two concatenated structures:
4//   1. BITMAPFILEHEADER (14 bytes, fixed):
5//      0  2  bfType       "BM" = 0x42 0x4D
6//      2  4  bfSize       total file size LE
7//      6  2  bfReserved1
8//      8  2  bfReserved2
9//      10 4  bfOffBits    offset of pixel data from file start
10//
11//   2. DIB header; the first u32 LE is its size, which discriminates:
12//       12  = BITMAPCOREHEADER (OS/2 1.x)
13//       40  = BITMAPINFOHEADER (Windows 3.0+, the common case)
14//       52  = BITMAPV2INFOHEADER
15//       56  = BITMAPV3INFOHEADER
16//       108 = BITMAPV4HEADER
17//       124 = BITMAPV5HEADER
18//
19// We parse the BMP file header and the first two u32s of the DIB
20// (its size + width), plus the canonical BITMAPINFOHEADER fields
21// when size >= 40.  Later DIB-version extensions are surfaced
22// only by their presence (size value).
23//
24// Invariants:
25//   B1  Magic "BM" (0x4D42 LE) required.
26//   B2  dib_size must be one of the canonical sizes.
27//   B3  Height may be negative in BITMAPINFOHEADER -- means top-down.
28//       We surface raw signed value; caller picks orientation.
29
30// nx_safety_envelope:
31//   intended_use:       AUTO_APPLIED -- primitive-specific tuning queued
32//   sil_target:         SIL1
33//   evidence:           [bulk_applied_2026-05-16, see-file-comment-for-detail]
34//   verdict:            NOT_YET_EVALUATED
35
36import "nx_syscalls.nx"
37
38const BMP_ERR_FORMAT: i64 = -1
39const BMP_ERR_SHORT:  i64 = -2
40const BMP_ERR_DIB:    i64 = -3
41
42struct BmpHeader {
43    file_size:     i64,
44    pixel_offset:  i64,
45    dib_size:      i64,
46    width:         i64,    // pixels
47    height:        i64,    // pixels; negative means top-down
48    planes:        i64,    // always 1
49    bits_per_pixel: i64,   // 1/4/8/16/24/32
50    compression:   i64,    // 0=BI_RGB, 1=BI_RLE8, ...
51    image_size:    i64,    // bytes; may be 0 for BI_RGB
52}
53
54func bmp_read_u16(buf: *u8, off: i64) -> i64 {
55    return buf[off] | (buf[off + 1] << 8)
56}
57
58func bmp_read_u32(buf: *u8, off: i64) -> i64 {
59    let b0: i64 = buf[off]
60    let b1: i64 = buf[off + 1]
61    let b2: i64 = buf[off + 2]
62    let b3: i64 = buf[off + 3]
63    return b0 | (b1 << 8) | (b2 << 16) | (b3 << 24)
64}
65
66// Sign-extend a 32-bit value into i64.
67func bmp_read_i32(buf: *u8, off: i64) -> i64 {
68    let v: i64 = bmp_read_u32(buf, off)
69    if v >= 0x80000000 {
70        return v - 0x100000000
71    }
72    return v
73}
74
75// Validate that a DIB size byte is one of the known versions.
76func bmp_dib_known(size: i64) -> i64 {
77    if size == 12 { return 1 }
78    if size == 40 { return 1 }
79    if size == 52 { return 1 }
80    if size == 56 { return 1 }
81    if size == 108 { return 1 }
82    if size == 124 { return 1 }
83    return 0
84}
85
86func bmp_parse_header(buf: *u8, n: i64, h: *BmpHeader) -> i64 {
87    if n < 14 + 40 { return BMP_ERR_SHORT }
88    if buf[0] != 0x42 { return BMP_ERR_FORMAT }   // 'B'
89    if buf[1] != 0x4D { return BMP_ERR_FORMAT }   // 'M'
90
91    h.file_size    = bmp_read_u32(buf, 2)
92    h.pixel_offset = bmp_read_u32(buf, 10)
93    h.dib_size     = bmp_read_u32(buf, 14)
94    if bmp_dib_known(h.dib_size) != 1 { return BMP_ERR_DIB }
95
96    if h.dib_size == 12 {
97        // BITMAPCOREHEADER: width/height are u16, planes/bpp follow.
98        h.width          = bmp_read_u16(buf, 18)
99        h.height         = bmp_read_u16(buf, 20)
100        h.planes         = bmp_read_u16(buf, 22)
101        h.bits_per_pixel = bmp_read_u16(buf, 24)
102        h.compression    = 0
103        h.image_size     = 0
104        return 0
105    }
106    // BITMAPINFOHEADER and later extensions share the first 40 bytes.
107    h.width          = bmp_read_i32(buf, 18)
108    h.height         = bmp_read_i32(buf, 22)
109    h.planes         = bmp_read_u16(buf, 26)
110    h.bits_per_pixel = bmp_read_u16(buf, 28)
111    h.compression    = bmp_read_u32(buf, 30)
112    h.image_size     = bmp_read_u32(buf, 34)
113    return 0
114}
115
116// Compile-only smoke -- 2x2 24-bit RGB BMP synth, file size 70
117// bytes (14 + 40 + 16 pixel-data).  Pixel offset = 54.
118func main() -> i64 {
119    let raw: *u8 = sys_mmap(128)
120    var i: i64 = 0
121    while i < 128 { raw[i] = 0; i = i + 1 }
122
123    raw[0] = 0x42; raw[1] = 0x4D
124    raw[2] = 70                  // file_size LE
125    raw[10] = 54                 // pixel_offset = 0x36
126    raw[14] = 40                 // dib_size = BITMAPINFOHEADER
127
128    raw[18] = 2                  // width = 2
129    raw[22] = 2                  // height = 2
130    raw[26] = 1                  // planes = 1
131    raw[28] = 24                 // bits_per_pixel = 24
132    raw[30] = 0                  // BI_RGB
133
134    let h_raw: *u8 = sys_mmap(128)
135    let h: *BmpHeader = h_raw as *BmpHeader
136    if bmp_parse_header(raw, 128, h) != 0 { return 1 }
137    if h.file_size != 70 { return 2 }
138    if h.pixel_offset != 54 { return 3 }
139    if h.dib_size != 40 { return 4 }
140    if h.width != 2 { return 5 }
141    if h.height != 2 { return 6 }
142    if h.bits_per_pixel != 24 { return 7 }
143
144    // Bad magic -> BMP_ERR_FORMAT.
145    raw[0] = 0
146    if bmp_parse_header(raw, 128, h) != BMP_ERR_FORMAT { return 8 }
147
148    // Unknown DIB size -> BMP_ERR_DIB.
149    raw[0] = 0x42
150    raw[14] = 99
151    if bmp_parse_header(raw, 128, h) != BMP_ERR_DIB { return 9 }
152    return 0
153}
nx_bmp_header.nx source

`nx_bmp_header.nx` source
