// Copyright 2019 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 version handles version types.
package version

import (
	"fmt"
	"regexp"
	"strings"

	"golang.org/x/mod/semver"
)

// Type defines the version types a module can have.
// This must be kept in sync with the 'version_type' database enum.
type Type string

const (
	// TypeRelease is a normal release.
	TypeRelease = Type("release")

	// TypePrerelease is a version with a prerelease.
	TypePrerelease = Type("prerelease")

	// TypePseudo appears to have a prerelease of the
	// form <commit date>-<commit hash>.
	TypePseudo = Type("pseudo")
)

func (t Type) String() string {
	return string(t)
}

var pseudoVersionRE = regexp.MustCompile(`^v[0-9]+\.(0\.0-|\d+\.\d+-([^+]*\.)?0\.)\d{14}-[A-Za-z0-9]+(\+incompatible)?$`)

// IsPseudo reports whether a valid version v is a pseudo-version.
// Modified from src/cmd/go/internal/modfetch.
func IsPseudo(v string) bool {
	return strings.Count(v, "-") >= 2 && pseudoVersionRE.MatchString(v)
}

// ParseType returns the Type of a given a version.
func ParseType(version string) (Type, error) {
	if !semver.IsValid(version) {
		return "", fmt.Errorf("ParseType(%q): invalid semver", version)
	}

	switch {
	case IsPseudo(version):
		return TypePseudo, nil
	case semver.Prerelease(version) != "":
		return TypePrerelease, nil
	default:
		return TypeRelease, nil
	}
}

// ForSorting returns a string that encodes version, so that comparing two such
// strings follows SemVer precedence, https://semver.org clause 11. It assumes
// version is valid. The returned string ends in '~' if and only if the version
// does not have a prerelease.
//
// For examples, see TestForSorting.
func ForSorting(version string) string {
	bytes := make([]byte, 0, len(version))
	prerelease := false // we are in the prerelease part
	nondigit := false   // this part has a non-digit character
	start := 1          // skip 'v'
	last := len(version)

	// Add the semver component version[start:end] to the result.
	addPart := func(end int) {
		if len(bytes) > 0 {
			// ',' comes before '-' and all letters and digits, so it correctly
			// imposes lexicographic ordering on the parts of the version.
			bytes = append(bytes, ',')
		}
		if nondigit {
			// Prepending the largest printable character '~' to a non-numeric
			// part, along with the fact that encoded numbers never begin with a
			// '~', (see appendNumericPrefix), ensures the semver requirement
			// that numeric identifiers always have lower precedence than
			// non-numeric ones.
			bytes = append(bytes, '~')
		} else {
			bytes = appendNumericPrefix(bytes, end-start)
		}
		bytes = append(bytes, version[start:end]...)
		start = end + 1 // skip over separator character
		nondigit = false
	}

loop:
	for i, c := range version[start:] {
		p := i + 1
		switch {
		case c == '.': // end of a part
			addPart(p)
		case c == '-': // first one is start of prerelease
			if !prerelease {
				prerelease = true
				addPart(p)
			} else {
				nondigit = true
			}
		case c == '+': // start of build; nothing after this matters
			last = p
			break loop

		case c < '0' || c > '9':
			nondigit = true
		}
	}
	if start < last {
		addPart(last)
	}
	if !prerelease {
		// Make sure prereleases appear first.
		bytes = append(bytes, '~')
	}
	return string(bytes)
}

// appendNumericPrefix appends a string representing n to dst.
// n is the length of a digit string; the value we append is a prefix for the
// digit string s such that
//   prefix1 + s1 < prefix2 + s2
// if and only if the integer denoted by s1 is less than the one denoted by s2.
// In other words, prefix + s is a string that can be compared with other such
// strings while preserving the ordering of the numbers.
//
// If n==1, there is no prefix. (Single-digit numbers are unchanged.)
// Otherwise, the prefix is a sequence of lower-case letters encoding n.
// Examples:
//   n    prefix
//   1    <none>
//   2    a
//   27   z
//   28   za
//   53   zz
//   54   zza
// This encoding depends on the ASCII properties that:
// - digits are ordered numerically
// - letters are ordered alphabetically
// - digits order before letters (so "1" < "a10")
func appendNumericPrefix(dst []byte, n int) []byte {
	n--
	for i := 0; i < n/26; i++ {
		dst = append(dst, 'z')
	}
	if rem := n % 26; rem > 0 {
		dst = append(dst, byte('a'+rem-1))
	}
	return dst
}