math: add MaxUint, MinInt, MaxInt

Since we have int8 to int64 min max and uint8 to uint64 max constants,
we should probably have some for the word size types too. This change
also adds tests to validate the correctness of all integer limit
values.

Fixes #28538

Change-Id: Idd25782e98d16c2abedf39959b7b66e9c4c0c98b
Reviewed-on: https://go-review.googlesource.com/c/go/+/247058
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Robert Griesemer <gri@golang.org>

Author: urandom2

src/math/const.go

@@ -37,6 +37,10 @@ const (
 
 // Integer limit values.
 const (
+	intSize = 32 << (^uint(0) >> 63) // 32 or 64
+
+	MaxInt    = 1<<(intSize-1) - 1
+	MinInt    = -1 << (intSize - 1)
 	MaxInt8   = 1<<7 - 1
 	MinInt8   = -1 << 7
 	MaxInt16  = 1<<15 - 1
@@ -45,6 +49,7 @@ const (
 	MinInt32  = -1 << 31
 	MaxInt64  = 1<<63 - 1
 	MinInt64  = -1 << 63
+	MaxUint   = 1<<intSize - 1
 	MaxUint8  = 1<<8 - 1
 	MaxUint16 = 1<<16 - 1
 	MaxUint32 = 1<<32 - 1

src/math/const_test.go

@@ -0,0 +1,47 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package math_test
+
+import (
+	"testing"
+
+	. "math"
+)
+
+func TestMaxUint(t *testing.T) {
+	if v := uint(MaxUint); v+1 != 0 {
+		t.Errorf("MaxUint should wrap around to zero: %d", v+1)
+	}
+	if v := uint8(MaxUint8); v+1 != 0 {
+		t.Errorf("MaxUint8 should wrap around to zero: %d", v+1)
+	}
+	if v := uint16(MaxUint16); v+1 != 0 {
+		t.Errorf("MaxUint16 should wrap around to zero: %d", v+1)
+	}
+	if v := uint32(MaxUint32); v+1 != 0 {
+		t.Errorf("MaxUint32 should wrap around to zero: %d", v+1)
+	}
+	if v := uint64(MaxUint64); v+1 != 0 {
+		t.Errorf("MaxUint64 should wrap around to zero: %d", v+1)
+	}
+}
+
+func TestMaxInt(t *testing.T) {
+	if v := int(MaxInt); v+1 != MinInt {
+		t.Errorf("MaxInt should wrap around to MinInt: %d", v+1)
+	}
+	if v := int8(MaxInt8); v+1 != MinInt8 {
+		t.Errorf("MaxInt8 should wrap around to MinInt8: %d", v+1)
+	}
+	if v := int16(MaxInt16); v+1 != MinInt16 {
+		t.Errorf("MaxInt16 should wrap around to MinInt16: %d", v+1)
+	}
+	if v := int32(MaxInt32); v+1 != MinInt32 {
+		t.Errorf("MaxInt32 should wrap around to MinInt32: %d", v+1)
+	}
+	if v := int64(MaxInt64); v+1 != MinInt64 {
+		t.Errorf("MaxInt64 should wrap around to MinInt64: %d", v+1)
+	}
+}