Add an id generator

Create UUID v4.

Create prefixed ids. To support more of the CDP protocol, we need to remove the
hard-coded IDs (session, browser context, frame, loader, ...) and be able to
dynamically create them, i.e. creating a new BrowserContextId when
Target.createBrowserContext is called.

var frame_id = id.Incremental(u16, "FRM"){};
frame_id.next() == "FRM-1"
frame_id.next() == "FRM-2"

Generation is allocation-free (the returned string is only valid until the
next call to next()). This is not thread safe, each CDP instance will have its
own generator (for each id it needs to generate).

The generated IDs are different than what Chrome uses, i.e.
BROWSERSESSIONID597D9875C664CAC0. I looked at various drivers and none have
any expectations beyond a string. Shorter IDs will be more efficient. Also, the
ID can cheeply be converted to and from an integer, allowing for lookups via
AutoHashMap(u16) instead of StringHashMap.

src/id.zig

@@ -0,0 +1,161 @@
+const std = @import("std");
+
+// Generates incrementing prefixed integers, i.e. CTX-1, CTX-2, CTX-3.
+// Wraps to 0 on overflow.
+// Many caveats for using this:
+// - Not thread-safe.
+// - Information leaking
+// - The slice returned by next() is only valid:
+//   - while incrementor is valid
+//   - until the next call to next()
+// On the positive, it's zero allocation
+fn Incrementing(comptime T: type, comptime prefix: []const u8) type {
+    // +1 for the '-' separator
+    const NUMERIC_START = prefix.len + 1;
+    const MAX_BYTES = NUMERIC_START + switch (T) {
+        u8 => 3,
+        u16 => 5,
+        u32 => 10,
+        u64 => 20,
+        else => @compileError("Incrementing must be given an unsigned int type, got: " ++ @typeName(T)),
+    };
+
+    const buffer = blk: {
+        var b = [_]u8{0} ** MAX_BYTES;
+        @memcpy(b[0..prefix.len], prefix);
+        b[prefix.len] = '-';
+        break :blk b;
+    };
+
+    const PrefixIntType = @Type(.{ .Int = .{
+        .bits = NUMERIC_START * 8,
+        .signedness = .unsigned,
+    } });
+
+    const PREFIX_INT_CODE: PrefixIntType = @bitCast(buffer[0..NUMERIC_START].*);
+
+    return struct {
+        current: T = 0,
+        buffer: [MAX_BYTES]u8 = buffer,
+
+        const Self = @This();
+
+        pub fn next(self: *Self) []const u8 {
+            const current = self.current;
+            const n = current +% 1;
+            defer self.current = n;
+
+            const size = std.fmt.formatIntBuf(self.buffer[NUMERIC_START..], n, 10, .lower, .{});
+            return self.buffer[0 .. NUMERIC_START + size];
+        }
+
+        // extracts the numeric portion from an ID
+        pub fn parse(str: []const u8) !T {
+            if (str.len <= NUMERIC_START) {
+                return error.InvalidId;
+            }
+
+            if (@as(PrefixIntType, @bitCast(str[0..NUMERIC_START].*)) != PREFIX_INT_CODE) {
+                return error.InvalidId;
+            }
+
+            return std.fmt.parseInt(T, str[NUMERIC_START..], 10) catch {
+                return error.InvalidId;
+            };
+        }
+    };
+}
+
+fn uuidv4(hex: []u8) void {
+    std.debug.assert(hex.len == 36);
+
+    var bin: [16]u8 = undefined;
+    std.crypto.random.bytes(&bin);
+    bin[6] = (bin[6] & 0x0f) | 0x40;
+    bin[8] = (bin[8] & 0x3f) | 0x80;
+
+    const alphabet = "0123456789abcdef";
+
+    hex[8] = '-';
+    hex[13] = '-';
+    hex[18] = '-';
+    hex[23] = '-';
+
+    const encoded_pos = [16]u8{ 0, 2, 4, 6, 9, 11, 14, 16, 19, 21, 24, 26, 28, 30, 32, 34 };
+    inline for (encoded_pos, 0..) |i, j| {
+        hex[i + 0] = alphabet[bin[j] >> 4];
+        hex[i + 1] = alphabet[bin[j] & 0x0f];
+    }
+}
+
+const hex_to_nibble = [_]u8{0xff} ** 48 ++ [_]u8{
+    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+    0x08, 0x09, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+    0xff, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0xff,
+    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+    0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+    0xff, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0xff,
+} ++ [_]u8{0xff} ** 152;
+
+const testing = std.testing;
+test "id: Incrementing.next" {
+    var id = Incrementing(u16, "IDX"){};
+    try testing.expectEqualStrings("IDX-1", id.next());
+    try testing.expectEqualStrings("IDX-2", id.next());
+    try testing.expectEqualStrings("IDX-3", id.next());
+
+    // force a wrap
+    id.current = 65533;
+    try testing.expectEqualStrings("IDX-65534", id.next());
+    try testing.expectEqualStrings("IDX-65535", id.next());
+    try testing.expectEqualStrings("IDX-0", id.next());
+}
+
+test "id: Incrementing.parse" {
+    const ReqId = Incrementing(u32, "REQ");
+    try testing.expectError(error.InvalidId, ReqId.parse(""));
+    try testing.expectError(error.InvalidId, ReqId.parse("R"));
+    try testing.expectError(error.InvalidId, ReqId.parse("RE"));
+    try testing.expectError(error.InvalidId, ReqId.parse("REQ"));
+    try testing.expectError(error.InvalidId, ReqId.parse("REQ-"));
+    try testing.expectError(error.InvalidId, ReqId.parse("REQ--1"));
+    try testing.expectError(error.InvalidId, ReqId.parse("REQ--"));
+    try testing.expectError(error.InvalidId, ReqId.parse("REQ-Nope"));
+    try testing.expectError(error.InvalidId, ReqId.parse("REQ-4294967296"));
+
+    try testing.expectEqual(0, try ReqId.parse("REQ-0"));
+    try testing.expectEqual(99, try ReqId.parse("REQ-99"));
+    try testing.expectEqual(4294967295, try ReqId.parse("REQ-4294967295"));
+}
+
+test "id: uuiv4" {
+    const expectUUID = struct {
+        fn expect(uuid: [36]u8) !void {
+            for (uuid, 0..) |b, i| {
+                switch (b) {
+                    '0'...'9', 'a'...'z' => {},
+                    '-' => {
+                        if (i != 8 and i != 13 and i != 18 and i != 23) {
+                            return error.InvalidEncoding;
+                        }
+                    },
+                    else => return error.InvalidHexEncoding,
+                }
+            }
+        }
+    }.expect;
+
+    var arena = std.heap.ArenaAllocator.init(testing.allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+
+    var seen = std.StringHashMapUnmanaged(void){};
+    for (0..100) |_| {
+        var hex: [36]u8 = undefined;
+        uuidv4(&hex);
+        try expectUUID(hex);
+        try seen.put(allocator, try allocator.dupe(u8, &hex), {});
+    }
+    try testing.expectEqual(100, seen.count());
+}