Implement and use generic approx equality tester (#8979)

Seems like there needs to be a general, easy-to-use solution for
approximate equality testing of containers holding floats (see, e.g.,
https://github.com/ghostty-org/ghostty/pull/8563#pullrequestreview-3281357931).
How's this?

src/datastruct/comparison.zig

@@ -0,0 +1,147 @@
+// The contents of this file is largely based on testing.zig from the Zig 0.15.1
+// stdlib, distributed under the MIT license, copyright (c) Zig contributors
+const std = @import("std");
+
+/// Generic, recursive equality testing utility using approximate comparison for
+/// floats and equality for everything else
+///
+/// Based on `std.testing.expectEqual` and `std.testing.expectEqualSlices`.
+///
+/// The relative tolerance is currently hardcoded to `sqrt(eps(float_type))`.
+pub inline fn expectApproxEqual(expected: anytype, actual: anytype) !void {
+    const T = @TypeOf(expected, actual);
+    return expectApproxEqualInner(T, expected, actual);
+}
+
+fn expectApproxEqualInner(comptime T: type, expected: T, actual: T) !void {
+    switch (@typeInfo(T)) {
+        // check approximate equality for floats
+        .float => {
+            const sqrt_eps = comptime std.math.sqrt(std.math.floatEps(T));
+            if (!std.math.approxEqRel(T, expected, actual, sqrt_eps)) {
+                print("expected approximately {any}, found {any}\n", .{ expected, actual });
+                return error.TestExpectedApproxEqual;
+            }
+        },
+
+        // recurse into containers
+        .array => {
+            const diff_index: usize = diff_index: {
+                const shortest = @min(expected.len, actual.len);
+                var index: usize = 0;
+                while (index < shortest) : (index += 1) {
+                    expectApproxEqual(actual[index], expected[index]) catch break :diff_index index;
+                }
+                break :diff_index if (expected.len == actual.len) return else shortest;
+            };
+            print("slices not approximately equal. first significant difference occurs at index {d} (0x{X})\n", .{ diff_index, diff_index });
+            return error.TestExpectedApproxEqual;
+        },
+        .vector => |info| {
+            var i: usize = 0;
+            while (i < info.len) : (i += 1) {
+                expectApproxEqual(expected[i], actual[i]) catch {
+                    print("index {d} incorrect. expected approximately {any}, found {any}\n", .{
+                        i, expected[i], actual[i],
+                    });
+                    return error.TestExpectedApproxEqual;
+                };
+            }
+        },
+        .@"struct" => |structType| {
+            inline for (structType.fields) |field| {
+                try expectApproxEqual(@field(expected, field.name), @field(actual, field.name));
+            }
+        },
+
+        // unwrap unions, optionals, and error unions
+        .@"union" => |union_info| {
+            if (union_info.tag_type == null) {
+                // untagged unions can only be compared bitwise,
+                // so expectEqual is all we need
+                std.testing.expectEqual(expected, actual) catch {
+                    return error.TestExpectedApproxEqual;
+                };
+            }
+
+            const Tag = std.meta.Tag(@TypeOf(expected));
+
+            const expectedTag = @as(Tag, expected);
+            const actualTag = @as(Tag, actual);
+
+            std.testing.expectEqual(expectedTag, actualTag) catch {
+                return error.TestExpectedApproxEqual;
+            };
+
+            // we only reach this switch if the tags are equal
+            switch (expected) {
+                inline else => |val, tag| try expectApproxEqual(val, @field(actual, @tagName(tag))),
+            }
+        },
+        .optional, .error_union => {
+            if (expected) |expected_payload| if (actual) |actual_payload| {
+                return expectApproxEqual(expected_payload, actual_payload);
+            };
+            // we only reach this point if there's at least one null or error,
+            // in which case expectEqual is all we need
+            std.testing.expectEqual(expected, actual) catch {
+                return error.TestExpectedApproxEqual;
+            };
+        },
+
+        // fall back to expectEqual for everything else
+        else => std.testing.expectEqual(expected, actual) catch {
+            return error.TestExpectedApproxEqual;
+        },
+    }
+}
+
+/// Copy of std.testing.print (not public)
+fn print(comptime fmt: []const u8, args: anytype) void {
+    if (@inComptime()) {
+        @compileError(std.fmt.comptimePrint(fmt, args));
+    } else if (std.testing.backend_can_print) {
+        std.debug.print(fmt, args);
+    }
+}
+
+// Tests based on the `expectEqual` tests in the Zig stdlib
+test "expectApproxEqual.union(enum)" {
+    const T = union(enum) {
+        a: i32,
+        b: f32,
+    };
+
+    const b10 = T{ .b = 10.0 };
+    const b10plus = T{ .b = 10.000001 };
+
+    try expectApproxEqual(b10, b10plus);
+}
+
+test "expectApproxEqual nested array" {
+    const a = [2][2]f32{
+        [_]f32{ 1.0, 0.0 },
+        [_]f32{ 0.0, 1.0 },
+    };
+
+    const b = [2][2]f32{
+        [_]f32{ 1.000001, 0.0 },
+        [_]f32{ 0.0, 0.999999 },
+    };
+
+    try expectApproxEqual(a, b);
+}
+
+test "expectApproxEqual vector" {
+    const a: @Vector(4, f32) = @splat(4.0);
+    const b: @Vector(4, f32) = @splat(4.000001);
+
+    try expectApproxEqual(a, b);
+}
+
+test "expectApproxEqual struct" {
+    const a = .{ 1, @as(f32, 1.0) };
+    const b = .{ 1, @as(f32, 0.999999) };
+
+    try expectApproxEqual(a, b);
+}

src/font/Collection.zig

@@ -19,6 +19,7 @@ const std = @import("std");
 const assert = std.debug.assert;
 const Allocator = std.mem.Allocator;
 const config = @import("../config.zig");
+const comparison = @import("../datastruct/comparison.zig");
 const font = @import("main.zig");
 const options = font.options;
 const DeferredFace = font.DeferredFace;
@@ -1199,7 +1200,7 @@ test "metrics" {
 
     try c.updateMetrics();
 
-    try std.testing.expectEqual(font.Metrics{
+    try comparison.expectApproxEqual(font.Metrics{
         .cell_width = 8,
         // The cell height is 17 px because the calculation is
         //
@@ -1229,12 +1230,12 @@ test "metrics" {
         .icon_height = 12.24,
         .face_width = 8.0,
         .face_height = 16.784,
-        .face_y = @round(3.04) - @as(f64, 3.04), // use f64, not comptime float, for exact match with runtime value
+        .face_y = -0.04,
     }, c.metrics);
 
     // Resize should change metrics
     try c.setSize(.{ .points = 24, .xdpi = 96, .ydpi = 96 });
-    try std.testing.expectEqual(font.Metrics{
+    try comparison.expectApproxEqual(font.Metrics{
         .cell_width = 16,
         .cell_height = 34,
         .cell_baseline = 6,
@@ -1249,7 +1250,7 @@ test "metrics" {
         .icon_height = 24.48,
         .face_width = 16.0,
         .face_height = 33.568,
-        .face_y = @round(6.08) - @as(f64, 6.08), // use f64, not comptime float, for exact match with runtime value
+        .face_y = -0.08,
     }, c.metrics);
 }
 
@@ -1493,29 +1494,7 @@ test "face metrics" {
         .{ narrowMetricsExpected, wideMetricsExpected },
         .{ narrowMetrics, wideMetrics },
     ) |metricsExpected, metricsActual| {
-        inline for (@typeInfo(font.Metrics.FaceMetrics).@"struct".fields) |field| {
-            const expected = @field(metricsExpected, field.name);
-            const actual = @field(metricsActual, field.name);
-            // Unwrap optional fields
-            const expectedValue, const actualValue = unwrap: switch (@typeInfo(field.type)) {
-                .optional => {
-                    if (expected) |expectedValue| if (actual) |actualValue| {
-                        break :unwrap .{ expectedValue, actualValue };
-                    };
-                    // Null values can be compared directly
-                    try std.testing.expectEqual(expected, actual);
-                    continue;
-                },
-                else => break :unwrap .{ expected, actual },
-            };
-            // All non-null values are floats
-            const eps = std.math.floatEps(@TypeOf(actualValue - expectedValue));
-            try std.testing.expectApproxEqRel(
-                expectedValue,
-                actualValue,
-                std.math.sqrt(eps),
-            );
-        }
+        try comparison.expectApproxEqual(metricsExpected, metricsActual);
     }
 
     // Verify estimated metrics. icWidth() should equal the smaller of