vpython: Add Python interpreter handling package.

vpython/python/version.go

Index: vpython/python/version.go
diff --git a/vpython/python/version.go b/vpython/python/version.go
new file mode 100644
index 0000000000000000000000000000000000000000..5d4639773bfc0166487c5b0c0333fd833a3dfc6a
--- /dev/null
+++ b/vpython/python/version.go
@@ -0,0 +1,144 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package python
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/luci/luci-go/common/errors"
+)
+
+// Version is a Python interpreter version.
+type Version struct {
+	Major int
+	Minor int
+	Patch int
+}
+
+// ParseVersion parses a Python version from a version string (e.g., "1.2.3").
+func ParseVersion(s string) (Version, error) {
+	if len(s) == 0 {
+		return Version{}, nil
+	}
+
+	parseVersion := func(value string) (int, error) {
+		version, err := strconv.Atoi(value)
+		if err != nil {
+			return 0, errors.Annotate(err).Reason("invalid number value: %(value)q").
+				D("value", value).
+				Err()
+		}
+		if version < 0 {
+			return 0, errors.Reason("version (%(version)d) must not be negative").
+				D("version", version).
+				Err()
+		}
+		return version, nil
+	}
+
+	var v Version
+	parts := strings.Split(s, ".")
+	var err error
+	switch l := len(parts); l {
+	case 3:
+		if v.Patch, err = parseVersion(parts[2]); err != nil {
+			return v, errors.Annotate(err).Reason("invalid patch value").Err()
+		}
+		fallthrough
+
+	case 2:
+		if v.Minor, err = parseVersion(parts[1]); err != nil {
+			return v, errors.Annotate(err).Reason("invalid minor value").Err()
+		}
+		fallthrough
+
+	case 1:
+		if v.Major, err = parseVersion(parts[0]); err != nil {
+			return v, errors.Annotate(err).Reason("invalid major value").Err()
+		}
+		if v.IsZero() {
+			return v, errors.Reason("version is incomplete").Err()
+		}
+		return v, nil
+
+	default:
+		return v, errors.Reason("unsupported number of parts (%(count)d)").
+			D("count", l).
+			Err()
+	}
+}
+
+func (v Version) String() string {
+	if v.IsZero() {
+		return ""
+	}
+	return fmt.Sprintf("%d.%d.%d", v.Major, v.Minor, v.Patch)
+}
+
+// IsZero returns true if the Version is empty. This is true if the Major field,
+// which must be set, is empty.
+func (v *Version) IsZero() bool { return v.Major <= 0 }
+
+// PythonBase returns the base Python interpreter name for this version.
+func (v *Version) PythonBase() string {
+	switch {
+	case v.IsZero():
+		return "python"
+	case v.Minor > 0:
+		return fmt.Sprintf("python%d.%d", v.Major, v.Minor)
+	default:
+		return fmt.Sprintf("python%d", v.Major)
+	}
+}
+
+// IsSatisfiedBy returns true if "other" is a suitable match for this version. A
+// suitable match:
+//
+//	- MUST have a Major version.
+//	- If v is zero, other is automatically suitable.
+//	- If v is non-zero, other must have the same Major version as v, and a
+//	  minor/patch version that is >= v's.
+func (v *Version) IsSatisfiedBy(other Version) bool {
+	switch {
+	case other.Major <= 0:
+		// "other" must have a Major version.
+		return false
+	case v.IsZero():
+		// "v" is zero (anything), so "other" satisfies it.
+		return true
+	case v.Major != other.Major:
+		// "other" must match "v"'s Major version precisely.
+		return false
+	case v.Minor > other.Minor:
+		// "v" requires a Minor version that is greater than "other"'s.
+		return false
+	case v.Minor < other.Minor:
+		// "v" requires a Minor version that is less than "other"'s.
+		return true
+	case v.Patch > other.Patch:
+		// "v" requires a Patch version that is greater than "other"'s.
+		return false
+	default:
+		return true
+	}
+}
+
+// Less returns true if "v"'s Version semantically precedes "other".
+func (v *Version) Less(other *Version) bool {
+	switch {
+	case v.Major < other.Major:
+		return true
+	case v.Major > other.Major:
+		return false
+	case v.Minor < other.Minor:
+		return true
+	case v.Minor > other.Minor:
+		return false
+	default:
+		return (v.Patch < other.Patch)
+	}
+}