// Copyright 2019 The Go Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// +build !go1.12

package span

import (
	"go/token"
)

// lineStart is the pre-Go 1.12 version of (*token.File).LineStart. For Go
// versions <= 1.11, we borrow logic from the analysisutil package.
// TODO(rstambler): Delete this file when we no longer support Go 1.11.
func lineStart(f *token.File, line int) token.Pos {
	// Use binary search to find the start offset of this line.

	min := 0        // inclusive
	max := f.Size() // exclusive
	for {
		offset := (min + max) / 2
		pos := f.Pos(offset)
		posn := f.Position(pos)
		if posn.Line == line {
			return pos - (token.Pos(posn.Column) - 1)
		}

		if min+1 >= max {
			return token.NoPos
		}

		if posn.Line < line {
			min = offset
		} else {
			max = offset
		}
	}
}