testing: stdlib examples

[mmcloughlin]

The standard library has a lot of examples of this kind of thing. Perhaps use them for test cases?

Related #2

[mmcloughlin]

$ find . -name '*.go' | xargs grep -El '[0-9]+\^[0-9]+'
./cmd/asm/internal/asm/expr_test.go
./cmd/compile/internal/gc/testdata/arith_test.go
./cmd/compile/internal/gc/universe.go
./cmd/link/internal/ld/data.go
./cmd/link/linkbig_test.go
./crypto/elliptic/p256_asm.go
./crypto/elliptic/p256_s390x.go
./crypto/elliptic/p256.go
./crypto/rsa/pss.go
./crypto/cipher/example_test.go
./crypto/tls/ticket.go
./crypto/rand/rand_linux.go
./crypto/ed25519/internal/edwards25519/edwards25519.go
./strconv/atof_test.go
./strconv/extfloat.go
./strconv/decimal.go
./strconv/atof.go
./strings/strings_test.go
./net/textproto/pipeline.go
./net/http/h2_bundle.go
./regexp/all_test.go
./bytes/bytes_test.go
./hash/adler32/adler32.go
./hash/crc32/gen_const_ppc64le.go
./runtime/error.go
./runtime/profbuf.go
./runtime/sema.go
./runtime/softfloat64.go
./encoding/asn1/asn1_test.go
./math/big/example_test.go
./math/big/nat.go
./math/big/nat_test.go
./math/big/prime_test.go
./math/rand/rand.go
./math/atan.go
./vendor/golang.org/x/crypto/poly1305/sum_generic.go
./vendor/golang.org/x/crypto/curve25519/curve25519.go
./vendor/golang.org/x/crypto/curve25519/mont25519_amd64.go
./vendor/golang.org/x/net/http2/hpack/hpack.go

For example:

$ find . -name '*.go' | xargs grep -E '[0-9]+\^[0-9]+' | head -n 20
./cmd/asm/internal/asm/expr_test.go:	{"3^4", 3 ^ 4, true},
./cmd/compile/internal/gc/testdata/arith_test.go:	v4 := (v1 * v1) ^ v1 | v1 - v1 - v1&v1 ^ uint8(3+2) + v1*1>>0 - v1 | 1 | v1<<(2*3|0-0*0^1)
./cmd/compile/internal/gc/testdata/arith_test.go:	v11 := 2&1 ^ 0 + 3 | int(0^0)<<1>>(1*0*3) ^ 0*0 ^ 3&0*3&3 ^ 3*3 ^ 1 ^ int(2)<<(2*3) + 2 | 2 | 2 ^ 2 + 1 | 3 | 0 ^ int(1)>>1 ^ 2 // int
./cmd/compile/internal/gc/universe.go:	maxfltval[TFLOAT32].SetString("33554431p103") // 2^24-1 p (127-23) + 1/2 ulp
./cmd/compile/internal/gc/universe.go:	maxfltval[TFLOAT64].SetString("18014398509481983p970") // 2^53-1 p (1023-52) + 1/2 ulp
./cmd/link/internal/ld/data.go:const cutoff = 2e9 // 2 GB (or so; looks better in errors than 2^31)
./cmd/link/internal/ld/data.go:	// On ppc64x a text section should not be larger than 2^26 bytes due to the size of
./cmd/link/linkbig_test.go:	// which is 2^26.  When that happens the call requires special trampolines or
./crypto/elliptic/p256_asm.go:	// This code operates in the Montgomery domain where R = 2^256 mod n
./crypto/elliptic/p256_asm.go:// p256Mul operates in a Montgomery domain with R = 2^256 mod p, where p is the
./crypto/elliptic/p256_s390x.go:	// This code operates in the Montgomery domain where R = 2^256 mod n
./crypto/elliptic/p256_s390x.go:// p256MulAsm operates in a Montgomery domain with R = 2^256 mod p, where p is the
./crypto/elliptic/p256_s390x.go:			0x75, 0xba, 0x95, 0xfc, 0x5f, 0xed, 0xb6, 0x01, 0x79, 0xe7, 0x30, 0xd4, 0x18, 0xa9, 0x14, 0x3c}, //(p256.x*2^256)%p
./crypto/elliptic/p256_s390x.go:			0x8b, 0x4a, 0xb8, 0xe4, 0xba, 0x19, 0xe4, 0x5c, 0xdd, 0xf2, 0x53, 0x57, 0xce, 0x95, 0x56, 0x0a}, //(p256.y*2^256)%p
./crypto/elliptic/p256_s390x.go:			0xff, 0xff, 0xff, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01}, //(p256.z*2^256)%p
./crypto/elliptic/p256.go:	p256Square(&ftmp, in)     // 2^1
./crypto/elliptic/p256.go:	p256Mul(&ftmp, in, &ftmp) // 2^2 - 2^0
./crypto/elliptic/p256.go:	p256Square(&ftmp, &ftmp)   // 2^3 - 2^1
./crypto/elliptic/p256.go:	p256Square(&ftmp, &ftmp)   // 2^4 - 2^2
./crypto/elliptic/p256.go:	p256Mul(&ftmp, &ftmp, &e2) // 2^4 - 2^0

[mmcloughlin]

$ find . -name '*.go' | xargs grep -Ec '\/\/.+[0-9]+\^[0-9]+' | awk -F: '$2 > 0 {print $2, $1}' | sort -nr | head
59 ./vendor/golang.org/x/crypto/curve25519/curve25519.go
50 ./crypto/ed25519/internal/edwards25519/edwards25519.go
45 ./strconv/extfloat.go
31 ./crypto/elliptic/p256.go
9 ./net/http/h2_bundle.go
6 ./strconv/atof.go
5 ./math/big/prime_test.go
5 ./crypto/elliptic/p256_s390x.go
4 ./math/big/nat.go
3 ./runtime/sema.go

[mmcloughlin]

probablyPrimeLucas in math/big/prime.go is another excellent example of where mathfmt could be used.

https://github.com/golang/go/blob/bfaf11c158b5ccd7db3c43727357b8b1071d7fde/src/math/big/prime.go#L126-L320