util: introduce helper for detecting file types

src/file_type.zig

@@ -0,0 +1,102 @@
+const std = @import("std");
+
+const type_details: []const struct {
+    typ: FileType,
+    sigs: []const []const ?u8,
+    exts: []const []const u8,
+} = &.{
+    .{
+        .typ = .jpeg,
+        .sigs = &.{
+            &.{ 0xFF, 0xD8, 0xFF, 0xDB },
+            &.{ 0xFF, 0xD8, 0xFF, 0xE0, 0x00, 0x10, 0x4A, 0x46, 0x49, 0x46, 0x00, 0x01 },
+            &.{ 0xFF, 0xD8, 0xFF, 0xEE },
+            &.{ 0xFF, 0xD8, 0xFF, 0xE1, null, null, 0x45, 0x78, 0x69, 0x66, 0x00, 0x00 },
+            &.{ 0xFF, 0xD8, 0xFF, 0xE0 },
+        },
+        .exts = &.{ ".jpg", ".jpeg", ".jfif" },
+    },
+    .{
+        .typ = .png,
+        .sigs = &.{&.{ 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A }},
+        .exts = &.{".png"},
+    },
+    .{
+        .typ = .gif,
+        .sigs = &.{
+            &.{ 'G', 'I', 'F', '8', '7', 'a' },
+            &.{ 'G', 'I', 'F', '8', '9', 'a' },
+        },
+        .exts = &.{".gif"},
+    },
+    .{
+        .typ = .bmp,
+        .sigs = &.{&.{ 'B', 'M' }},
+        .exts = &.{".bmp"},
+    },
+    .{
+        .typ = .qoi,
+        .sigs = &.{&.{ 'q', 'o', 'i', 'f' }},
+        .exts = &.{".qoi"},
+    },
+    .{
+        .typ = .webp,
+        .sigs = &.{
+            &.{ 0x52, 0x49, 0x46, 0x46, null, null, null, null, 0x57, 0x45, 0x42, 0x50 },
+        },
+        .exts = &.{".webp"},
+    },
+};
+
+/// This is a helper for detecting file types based on magic bytes.
+///
+/// Ref: https://en.wikipedia.org/wiki/List_of_file_signatures
+pub const FileType = enum {
+    /// JPEG image file.
+    jpeg,
+
+    /// PNG image file.
+    png,
+
+    /// GIF image file.
+    gif,
+
+    /// BMP image file.
+    bmp,
+
+    /// QOI image file.
+    qoi,
+
+    /// WebP image file.
+    webp,
+
+    /// Unknown file format.
+    unknown,
+
+    /// Detect file type based on the magic bytes
+    /// at the start of the provided file contents.
+    pub fn detect(contents: []const u8) FileType {
+        inline for (type_details) |typ| {
+            inline for (typ.sigs) |signature| {
+                if (contents.len >= signature.len) {
+                    for (contents[0..signature.len], signature) |f, sig| {
+                        if (sig) |s| if (f != s) break;
+                    } else {
+                        return typ.typ;
+                    }
+                }
+            }
+        }
+        return .unknown;
+    }
+
+    /// Guess file type from its extension.
+    pub fn guessFromExtension(extension: []const u8) FileType {
+        inline for (type_details) |typ| {
+            inline for (typ.exts) |ext| {
+                if (std.ascii.eqlIgnoreCase(extension, ext)) return typ.typ;
+            }
+        }
+        return .unknown;
+    }
+};