package auth

import (
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"strings"
	"testing"
)

// 6. Session tokens carry real entropy from crypto/rand.
//
// Randomness cannot be proved by a test, so this asserts the properties whose
// absence would mean the generator is broken: the full 256 bits are present,
// the output is not a constant, and the bytes are not all the same value —
// which is what a zeroed or unseeded buffer looks like.
func TestGenerateTokenIsCryptographicallyRandom(t *testing.T) {
	const runs = 512

	seen := make(map[string]struct{}, runs)
	bitsSet := make([]int, 8*TokenBytes) // how often each bit position was 1

	for i := 0; i < runs; i++ {
		token, err := GenerateToken()
		if err != nil {
			t.Fatalf("GenerateToken: %v", err)
		}

		raw, err := base64.RawURLEncoding.DecodeString(token)
		if err != nil {
			t.Fatalf("token is not base64url: %v", err)
		}
		if len(raw) != TokenBytes {
			t.Fatalf("token decodes to %d bytes, want %d", len(raw), TokenBytes)
		}
		// A cookie value must survive a round trip untouched: no '=' padding,
		// no '+' or '/' to be re-encoded.
		if strings.ContainsAny(token, "=+/") {
			t.Fatalf("token contains a character that is unsafe in a cookie or URL")
		}

		if _, dup := seen[token]; dup {
			t.Fatalf("GenerateToken returned a duplicate within %d calls", runs)
		}
		seen[token] = struct{}{}

		for bit := 0; bit < 8*TokenBytes; bit++ {
			if raw[bit/8]&(1<<(bit%8)) != 0 {
				bitsSet[bit]++
			}
		}
	}

	// Each bit should be 1 about half the time. A bit that is *always* 0 or
	// always 1 across 512 draws has a chance of roughly 2^-511 of being random
	// and is far more likely a stuck generator. The bound is deliberately
	// loose — this is a smoke test for a broken source, not a statistical
	// suite, and it must never flake.
	for bit, count := range bitsSet {
		if count == 0 || count == runs {
			t.Errorf("bit %d was constant across %d tokens; the entropy source is broken", bit, runs)
		}
	}

	// 256 bits is the size that makes guessing a live session hopeless.
	if TokenBytes < 32 {
		t.Errorf("TokenBytes = %d, want at least 32", TokenBytes)
	}
}

// 7. Two generated tokens differ.
func TestGenerateTokenReturnsDistinctValues(t *testing.T) {
	a, err := GenerateToken()
	if err != nil {
		t.Fatalf("first: %v", err)
	}
	b, err := GenerateToken()
	if err != nil {
		t.Fatalf("second: %v", err)
	}
	if a == b {
		t.Fatal("two consecutive tokens were identical")
	}
	if HashToken(a) == HashToken(b) {
		t.Fatal("two distinct tokens hashed to the same value")
	}
}

// 8. Hashing is deterministic, and is genuinely SHA-256 rather than something
// that merely looks like it.
func TestHashTokenIsDeterministicSHA256(t *testing.T) {
	token, err := GenerateToken()
	if err != nil {
		t.Fatalf("GenerateToken: %v", err)
	}

	first, second := HashToken(token), HashToken(token)
	if first != second {
		t.Fatal("hashing the same token twice produced different values")
	}

	// Checked against the standard library directly: lookup only works if the
	// value stored is exactly this.
	want := sha256.Sum256([]byte(token))
	if first != hex.EncodeToString(want[:]) {
		t.Fatal("HashToken does not agree with crypto/sha256")
	}

	if len(first) != 64 {
		t.Fatalf("hash is %d characters, want 64 hex characters", len(first))
	}
	if first != strings.ToLower(first) {
		t.Error("hash is not lowercase; the database CHECK requires lowercase hex")
	}
	// The stored value must not be the secret.
	if strings.Contains(first, token) || first == token {
		t.Error("the hash contains the token")
	}
	if HashToken(token+"x") == first {
		t.Error("a different token hashed to the same value")
	}

	// The empty string has a hash too — that is a property of SHA-256, not a
	// licence to store one. Guarding against an empty token is the Manager's
	// job, and TestManagerRejectsEmptyToken covers it.
	if HashToken("") == "" {
		t.Error("HashToken returned an empty string")
	}
}

// IsTokenHash is the guard that stops a raw token being written where a hash
// belongs, so it must reject every raw token and accept every real hash.
func TestIsTokenHash(t *testing.T) {
	token, err := GenerateToken()
	if err != nil {
		t.Fatalf("GenerateToken: %v", err)
	}

	if !IsTokenHash(HashToken(token)) {
		t.Error("a real hash was not recognised as one")
	}
	if IsTokenHash(token) {
		t.Error("a raw token was accepted as a hash; this is the check that prevents storing the secret")
	}

	for name, s := range map[string]string{
		"empty":     "",
		"too short": strings.Repeat("a", 63),
		"too long":  strings.Repeat("a", 65),
		"uppercase": strings.ToUpper(HashToken(token)),
		"non-hex":   strings.Repeat("g", 64),
		"trailing":  HashToken(token) + "\n",
	} {
		if IsTokenHash(s) {
			t.Errorf("%s was accepted as a token hash", name)
		}
	}
}