From 09882358129ac88cecdea8808a47c3d3d7903d02 Mon Sep 17 00:00:00 2001
From: Meghan Denny <hello@nektro.net>
Date: Mon, 2 Mar 2026 13:41:50 -0800
Subject: [PATCH] add decodeBase32

---
 test.zig |  8 ++++++++
 totp.zig | 22 ++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/test.zig b/test.zig
index b1bc6394085a6c3e00f9c3ac28c6d38ae83b227c..4aa8362ad1b227909745a358a6f9009cb1f5c568 100644
--- a/test.zig
+++ b/test.zig
@@ -59,3 +59,11 @@ test {
     defer allocator.free(url);
     try expect(url).toEqualString("otpauth://totp/otpauth%20demo:username@example.org?secret=AAAABBBB22223333YYYYZZZZ66667777&algorithm=SHA1&digits=6&period=30&issuer=otpauth%20demo");
 }
+
+test {
+    const b32 = "SC5IH4S3HJLEWOGXJM4BL6CH5BDSYHBV";
+    const hex_expected = "90ba83f25b3a564b38d74b3815f847e8472c1c35";
+    const raw_actual = try totp.decodeBase32(b32, std.crypto.hash.Sha1);
+    const hex_actual = extras.to_hex(raw_actual);
+    try expect(&hex_actual).toEqualString(hex_expected);
+}
diff --git a/totp.zig b/totp.zig
index 2396a2659c300e384b93159fc87fb541c6033911..2739d35ea82c3a60ca69bd6dffb7961076882fed 100644
--- a/totp.zig
+++ b/totp.zig
@@ -123,3 +123,25 @@ const BufBiterator = struct {
         return @intFromBool(result);
     }
 };
+
+pub fn decodeBase32(input: []const u8, Hash: type) ![Hash.digest_length]u8 {
+    var result: std.bit_set.ArrayBitSet(u8, Hash.digest_length * 8) = .initEmpty();
+    const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
+    for (input, 0..) |c, i| {
+        const x = std.mem.indexOfScalar(u8, alphabet, c) orelse return error.InvalidCharacter;
+        const n: u5 = @intCast(x);
+        inline for (0..5) |j| blk: {
+            if (i * 5 > result.capacity()) return error.Overflow;
+            const d = i * 5 + j;
+            if (d >= result.capacity()) break :blk;
+            result.setValue(d, n & (1 << 5 - 1 - (j)) != 0);
+        }
+    }
+    return bitReverse(result.masks);
+}
+
+fn bitReverse(array: anytype) [array.len]std.meta.Child(@TypeOf(array)) {
+    var result: [array.len]std.meta.Child(@TypeOf(array)) = @splat(0);
+    for (&array, &result) |x, *y| y.* = @bitReverse(x);
+    return result;
+}
-- 
2.54.0