Adding Base32 decode method

It can currently decode when the encoded string is a multiple of 8
bytes.

src/base32.zig

@@ -0,0 +1,84 @@
+const std = @import("std");
+const expect = std.testing.expect;
+const expectEqualStrings = std.testing.expectEqualStrings;
+const Allocator = std.mem.Allocator;
+
+const Base32Error = error{InvalidLength};
+
+const Base32 = struct {
+    ///
+    pub fn decodeU8(allocator: Allocator, data: []const u8) ![]const u8 {
+        const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
+
+        if (data.len % 8 != 0) {
+            return error.InvalidLength;
+        }
+
+        const result = try allocator.alloc(u8, (data.len / 8) * 5);
+
+        var i: u64 = 0;
+        var r: usize = 0;
+        while (i < data.len) {
+            var bytes: u40 = 0;
+            const v1: u8 = @truncate(std.mem.indexOfScalar(u8, alphabet, data[i]).?);
+            const v2: u8 = @truncate(std.mem.indexOfScalar(u8, alphabet, data[i + 1]).?);
+            const v3: u8 = @truncate(std.mem.indexOfScalar(u8, alphabet, data[i + 2]).?);
+            const v4: u8 = @truncate(std.mem.indexOfScalar(u8, alphabet, data[i + 3]).?);
+            const v5: u8 = @truncate(std.mem.indexOfScalar(u8, alphabet, data[i + 4]).?);
+            const v6: u8 = @truncate(std.mem.indexOfScalar(u8, alphabet, data[i + 5]).?);
+            const v7: u8 = @truncate(std.mem.indexOfScalar(u8, alphabet, data[i + 6]).?);
+            const v8: u8 = @truncate(std.mem.indexOfScalar(u8, alphabet, data[i + 7]).?);
+            bytes = v1;
+            bytes = bytes << 5 | v2;
+            bytes = bytes << 5 | v3;
+            bytes = bytes << 5 | v4;
+            bytes = bytes << 5 | v5;
+            bytes = bytes << 5 | v6;
+            bytes = bytes << 5 | v7;
+            bytes = bytes << 5 | v8;
+            i += 8;
+
+            result[r] = @as(u8, @truncate((bytes >> 32) & 0b11111111));
+            r += 1;
+            result[r] = @as(u8, @truncate((bytes >> 24) & 0b11111111));
+            r += 1;
+            result[r] = @as(u8, @truncate((bytes >> 16) & 0b11111111));
+            r += 1;
+            result[r] = @as(u8, @truncate((bytes >> 8) & 0b11111111));
+            r += 1;
+            result[r] = @as(u8, @truncate((bytes) & 0b11111111));
+            r += 1;
+        }
+
+        return result;
+    }
+};
+
+test "base32 decode base32('abcde') " {
+    try testDecode("abcde", "MFRGGZDF");
+}
+test "base32 decode base32('aaaaa') " {
+    try testDecode("aaaaa", "MFQWCYLB");
+}
+
+fn testDecode(expected_decoded: []const u8, encoded: []const u8) !void {
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    const allocator = gpa.allocator();
+
+    const decoded = try Base32.decodeU8(allocator, encoded);
+    try expectEqualStrings(expected_decoded, decoded);
+}
+
+test "base32 decode - invalid length" {
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    const allocator = gpa.allocator();
+
+    try std.testing.expect(Base32.decodeU8(allocator, "1") == error.InvalidLength);
+    try std.testing.expect(Base32.decodeU8(allocator, "12") == error.InvalidLength);
+    try std.testing.expect(Base32.decodeU8(allocator, "123") == error.InvalidLength);
+    try std.testing.expect(Base32.decodeU8(allocator, "1234") == error.InvalidLength);
+    try std.testing.expect(Base32.decodeU8(allocator, "12345") == error.InvalidLength);
+    try std.testing.expect(Base32.decodeU8(allocator, "123456") == error.InvalidLength);
+    try std.testing.expect(Base32.decodeU8(allocator, "1234567") == error.InvalidLength);
+    try std.testing.expect(Base32.decodeU8(allocator, "123456789") == error.InvalidLength);
+}