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// Package version tells the editor what build of itself it is.
//
// The number comes from whichever of three sources knows it, in this order:
// the linker, the Go build system, and a constant of last resort. Nothing here
// reads a file or runs a command, so it costs nothing to ask.
package version

import (
	"runtime/debug"
	"strings"
	"time"
)

// The build stamps these through the linker:
//
//	go build -ldflags "-X 'rickub.com/turbo-editors/turbo-core/version.stamp=v0.2.0'" .
//
// The Makefile and scripts/install.sh both do it, filling stamp from
// `git describe --tags --dirty`. They are unexported because nothing but the
// linker should ever write them, and package-level strings are what -X can set.
var (
	stamp  string
	commit string
	built  string
)

// unknownNumber is what a build reports when no source could name it. It is
// deliberately not a version number: a stale one printed as fact is the defect
// this package exists to remove.
const unknownNumber = "unknown"

// develVersion is what the Go build system calls a binary built from a working
// tree when it can read no version at all.
const develVersion = "(devel)"

// pseudoTailLength is the width of the tail every Go pseudo-version ends with:
// a dash, a 14-digit UTC timestamp, a dash, and twelve hex characters.
const pseudoTailLength = 1 + 14 + 1 + 12

// shortCommitLength is how much of a revision hash is worth showing. Seven
// characters is what git itself abbreviates to, and what a reader will paste
// back into `git show`.
const shortCommitLength = 7

// Info is what a binary knows about its own build.
//
// Commit and Built are empty when nothing recorded them, which is normal: a
// binary from `go install …@v0.2.0` knows its version and nothing else. A
// caller shows the fields it has and says nothing about the rest.
type Info struct {
	// Number is the version, without a leading "v": "0.2.0" for a release,
	// "0.1.0-14-g88a4c38" for a build between two of them.
	Number string
	// Commit is the abbreviated revision, when it is known separately from
	// Number.
	Commit string
	// Built is when the binary was linked, in RFC 3339, when it is known.
	Built string
}

// Current returns what this binary knows about itself.
//
//	info := version.Current()
//	fmt.Println(info.Number) // "0.1.0-14-g88a4c38"
func Current() Info {
	return resolve(stamp, commit, built, readBuildInfo())
}

// String renders the whole of what is known on one line, for `-version`.
//
// One line rather than several because the installer prints it beside a path,
// and because a caller that wants the parts has the fields.
//
//	version.Info{Number: "0.2.0", Commit: "88a4c38"}.String()
//	// "0.2.0 (88a4c38)"
func (i Info) String() string {
	switch {
	case i.Commit != "" && i.Built != "":
		return i.Number + " (" + i.Commit + ", built " + i.Built + ")"
	case i.Commit != "":
		return i.Number + " (" + i.Commit + ")"
	case i.Built != "":
		return i.Number + " (built " + i.Built + ")"
	}
	return i.Number
}

// resolve turns the raw build facts into an Info.
//
// It is separate from Current so that every combination can be tested without
// linker flags: the interesting cases are precisely the ones a test binary
// cannot be built into.
func resolve(stamp, commit, built string, info *debug.BuildInfo) Info {
	if stamp != "" {
		return Info{Number: trimV(stamp), Commit: shorten(commit), Built: built}
	}
	if info == nil {
		return Info{Number: unknownNumber}
	}
	return fromBuildInfo(info)
}

// fromBuildInfo reads what the Go build system recorded.
//
// Two quite different builds arrive here. `go install <module>@v0.2.0` records
// the module version and **no** VCS information at all, because there was no
// repository to read. A plain `go build .` in a checkout records the reverse:
// the version is "(devel)" and the revision is known. Neither can name a tag —
// the build system does not read them — which is why a build that wants to
// show one has to be stamped.
func fromBuildInfo(info *debug.BuildInfo) Info {
	settings := settingsOf(info)
	out := Info{
		Number: trimV(info.Main.Version),
		Commit: shorten(settings["vcs.revision"]),
	}

	// A pseudo-version is the Go tool naming a commit that no tag names —
	// "0.1.1-0.20260831165958-88a4c3859bf3+dirty". It is unreadable in a
	// dialog, and its "0.1.1" is a patch release that does not exist, so it is
	// reported for what it means rather than for what it says.
	if out.Number == "" || out.Number == develVersion || isPseudoVersion(out.Number) {
		out.Number = develNumber(settings)
	}
	// vcs.time is when the *commit* was made, not when this binary was linked,
	// so it is not a build date and is not reported as one.
	return out
}

// develNumber names a build made from a working tree, where no tag is
// available: "devel", and "devel-dirty" when the tree had uncommitted changes.
func develNumber(settings map[string]string) string {
	if settings["vcs.modified"] == "true" {
		return "devel-dirty"
	}
	if _, built := settings["vcs.revision"]; built {
		return "devel"
	}
	return unknownNumber
}

// isPseudoVersion reports whether a module version is one the Go tool invented
// for a commit that no tag names.
//
// Every form of one ends the same way, whatever base version precedes it, and
// build metadata such as "+dirty" is not part of that tail.
//
//	isPseudoVersion("0.1.1-0.20260831165958-88a4c3859bf3+dirty") // true
//	isPseudoVersion("0.2.0")                                     // false
func isPseudoVersion(number string) bool {
	number = withoutBuildMetadata(number)
	if len(number) <= pseudoTailLength {
		return false // nothing left for the base version it was derived from
	}
	return hasPseudoTail(number[len(number)-pseudoTailLength:])
}

// hasPseudoTail reports whether a tail of exactly pseudoTailLength characters
// is the "-yyyymmddhhmmss-abcdefabcdef" a pseudo-version ends with.
func hasPseudoTail(tail string) bool {
	if !isAll(tail[1:15], isDigit) || !isAll(tail[16:], isHex) {
		return false
	}
	// The character before the timestamp is a dash when no tag precedes the
	// commit ("v0.0.0-<ts>-<hash>") and a dot when one does, because the base
	// then ends in "-0." or "-pre.0.".
	separatesTimestamp := tail[0] == '-' || tail[0] == '.'
	return separatesTimestamp && tail[15] == '-'
}

// withoutBuildMetadata drops the "+dirty" or "+incompatible" a version may
// carry, which is not part of the pseudo-version tail.
func withoutBuildMetadata(number string) string {
	if plus := strings.IndexByte(number, '+'); plus >= 0 {
		return number[:plus]
	}
	return number
}

// isAll reports whether every byte of s satisfies want.
func isAll(s string, want func(byte) bool) bool {
	for i := 0; i < len(s); i++ {
		if !want(s[i]) {
			return false
		}
	}
	return true
}

func isDigit(c byte) bool { return c >= '0' && c <= '9' }

func isHex(c byte) bool { return isDigit(c) || (c >= 'a' && c <= 'f') }

// settingsOf turns a build's settings into a map, which is how every caller
// here wants them.
func settingsOf(info *debug.BuildInfo) map[string]string {
	out := make(map[string]string, len(info.Settings))
	for _, setting := range info.Settings {
		out[setting.Key] = setting.Value
	}
	return out
}

// trimV drops the leading "v" a git tag carries. The tag is `v0.2.0`; what a
// person reads in an About box is `0.2.0`.
func trimV(number string) string {
	if len(number) > 1 && number[0] == 'v' && number[1] >= '0' && number[1] <= '9' {
		return number[1:]
	}
	return number
}

// shorten abbreviates a revision hash the way git does.
func shorten(revision string) string {
	if len(revision) <= shortCommitLength {
		return revision
	}
	return revision[:shortCommitLength]
}

// BuiltAt renders Built for a person: "2026-08-31 18:04 UTC".
//
// A stamp that does not parse is shown as it is rather than dropped — a build
// whose date is malformed should say so, not appear to have none.
//
//	version.Info{Built: "2026-08-31T18:04:05Z"}.BuiltAt() // "2026-08-31 18:04 UTC"
func (i Info) BuiltAt() string {
	if i.Built == "" {
		return ""
	}
	moment, err := time.Parse(time.RFC3339, i.Built)
	if err != nil {
		return i.Built
	}
	return moment.UTC().Format("2006-01-02 15:04") + " UTC"
}

// readBuildInfo is debug.ReadBuildInfo, with the "ok" folded into a nil.
func readBuildInfo() *debug.BuildInfo {
	info, ok := debug.ReadBuildInfo()
	if !ok {
		return nil
	}
	return info
}