Improve pagination (#105)

vendor/golang.org/x/tools/internal/span/utf16.go

@@ -0,0 +1,94 @@
+// 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 span
+
+import (
+	"fmt"
+	"unicode/utf16"
+	"unicode/utf8"
+)
+
+// ToUTF16Column calculates the utf16 column expressed by the point given the
+// supplied file contents.
+// This is used to convert from the native (always in bytes) column
+// representation and the utf16 counts used by some editors.
+func ToUTF16Column(p Point, content []byte) (int, error) {
+	if content == nil {
+		return -1, fmt.Errorf("ToUTF16Column: missing content")
+	}
+	if !p.HasPosition() {
+		return -1, fmt.Errorf("ToUTF16Column: point is missing position")
+	}
+	if !p.HasOffset() {
+		return -1, fmt.Errorf("ToUTF16Column: point is missing offset")
+	}
+	offset := p.Offset()      // 0-based
+	colZero := p.Column() - 1 // 0-based
+	if colZero == 0 {
+		// 0-based column 0, so it must be chr 1
+		return 1, nil
+	} else if colZero < 0 {
+		return -1, fmt.Errorf("ToUTF16Column: column is invalid (%v)", colZero)
+	}
+	// work out the offset at the start of the line using the column
+	lineOffset := offset - colZero
+	if lineOffset < 0 || offset > len(content) {
+		return -1, fmt.Errorf("ToUTF16Column: offsets %v-%v outside file contents (%v)", lineOffset, offset, len(content))
+	}
+	// Use the offset to pick out the line start.
+	// This cannot panic: offset > len(content) and lineOffset < offset.
+	start := content[lineOffset:]
+
+	// Now, truncate down to the supplied column.
+	start = start[:colZero]
+
+	// and count the number of utf16 characters
+	// in theory we could do this by hand more efficiently...
+	return len(utf16.Encode([]rune(string(start)))) + 1, nil
+}
+
+// FromUTF16Column advances the point by the utf16 character offset given the
+// supplied line contents.
+// This is used to convert from the utf16 counts used by some editors to the
+// native (always in bytes) column representation.
+func FromUTF16Column(p Point, chr int, content []byte) (Point, error) {
+	if !p.HasOffset() {
+		return Point{}, fmt.Errorf("FromUTF16Column: point is missing offset")
+	}
+	// if chr is 1 then no adjustment needed
+	if chr <= 1 {
+		return p, nil
+	}
+	if p.Offset() >= len(content) {
+		return p, fmt.Errorf("FromUTF16Column: offset (%v) greater than length of content (%v)", p.Offset(), len(content))
+	}
+	remains := content[p.Offset():]
+	// scan forward the specified number of characters
+	for count := 1; count < chr; count++ {
+		if len(remains) <= 0 {
+			return Point{}, fmt.Errorf("FromUTF16Column: chr goes beyond the content")
+		}
+		r, w := utf8.DecodeRune(remains)
+		if r == '\n' {
+			// Per the LSP spec:
+			//
+			// > If the character value is greater than the line length it
+			// > defaults back to the line length.
+			break
+		}
+		remains = remains[w:]
+		if r >= 0x10000 {
+			// a two point rune
+			count++
+			// if we finished in a two point rune, do not advance past the first
+			if count >= chr {
+				break
+			}
+		}
+		p.v.Column += w
+		p.v.Offset += w
+	}
+	return p, nil
+}