From dd39cdcf27a4d8c2c0472f4cab2eb96fc80cf8b7 Mon Sep 17 00:00:00 2001
From: Nathan Reiner <nathan@nathanreiner.xyz>
Date: Wed, 18 Mar 2026 23:44:08 +0100
Subject: jpg: add JFIF segment for APP0 marker

Automatically parse APP0 as a JFIF segment.
The following does not yet work:
  * The extended JFIF segment marked by `JFXX` is not recognized.
  * The thumbnail is not loaded but tossed, so it cannot be used
    currently
---
 src/jpg/jfif.zig | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)
 create mode 100644 src/jpg/jfif.zig

(limited to 'src/jpg/jfif.zig')

diff --git a/src/jpg/jfif.zig b/src/jpg/jfif.zig
new file mode 100644
index 0000000..91c95df
--- /dev/null
+++ b/src/jpg/jfif.zig
@@ -0,0 +1,52 @@
+const std = @import("std");
+
+pub const Version = packed struct {
+    major: u8,
+    minor: u8,
+
+    pub fn format(self: @This(), writer: *std.Io.Writer) !void {
+        try writer.print("v{}.{}", .{self.major, self.minor});
+    }
+};
+
+pub const Unit = enum(u8) {
+    none = 0,
+    pixels_per_inch = 1,
+    pixels_per_cm = 2,
+};
+
+const Self = @This();
+
+version: Version,
+unit: Unit,
+density: struct { x: u16, y: u16 },
+thumbnail: struct { x: u8, y: u8 },
+
+const identifier: []const u8 = "JFIF\x00";
+
+pub fn read(reader: *std.Io.Reader) !Self {
+    const ident = try reader.peekArray(identifier.len);
+
+    if (!std.mem.eql(u8, ident, identifier)) {
+        return error.WrongIdentifier;
+    }
+    reader.toss(identifier.len);
+
+    var self: Self = undefined;
+    self.version.major = try reader.takeByte();
+    self.version.minor = try reader.takeByte();
+
+    self.unit = @enumFromInt(try reader.takeByte());
+
+    self.density.x = try reader.takeInt(u16, .big);
+    self.density.y = try reader.takeInt(u16, .big);
+
+    self.thumbnail.x = try reader.takeInt(u8, .big);
+    self.thumbnail.y = try reader.takeInt(u8, .big);
+
+    // WARNING: we do not decode the thumbnail yet.
+    const size = 3 * self.thumbnail.x * self.thumbnail.y;
+    reader.toss(size);
+
+    return self;
+}
-- 
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