internal/lsp: add compare functions for spans

Needed in order to sort lists of spans for stable tests, also used for span
equality tests.

Change-Id: Id2bedff4d7136494e2302c02bcb9bdc662ccfeb1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/172660
Run-TryBot: Ian Cottrell <iancottrell@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/span/span.go

@@ -7,6 +7,7 @@ package span
 import (
 	"encoding/json"
 	"fmt"
+	"strings"
 )
 
 // Span represents a source code range in standardized form.
@@ -56,6 +57,45 @@ func NewPoint(line, col, offset int) Point {
 	return p
 }
 
+func Compare(a, b Span) int {
+	if r := strings.Compare(string(a.v.URI), string(b.v.URI)); r != 0 {
+		return r
+	}
+	if r := comparePoint(a.v.Start, b.v.Start); r != 0 {
+		return r
+	}
+	return comparePoint(a.v.End, b.v.End)
+}
+
+func ComparePoint(a, b Point) int {
+	return comparePoint(a.v, b.v)
+}
+
+func comparePoint(a, b point) int {
+	if !a.hasPosition() {
+		if a.Offset < b.Offset {
+			return -1
+		}
+		if a.Offset > b.Offset {
+			return 1
+		}
+		return 0
+	}
+	if a.Line < b.Line {
+		return -1
+	}
+	if a.Line > b.Line {
+		return 1
+	}
+	if a.Column < b.Column {
+		return -1
+	}
+	if a.Column > b.Column {
+		return 1
+	}
+	return 0
+}
+
 func (s Span) HasPosition() bool             { return s.v.Start.hasPosition() }
 func (s Span) HasOffset() bool               { return s.v.Start.hasOffset() }
 func (s Span) IsValid() bool                 { return s.v.Start.isValid() }