Add support for a relative offset within ast.SourceInfo (#836)

Author: TristonianJones

common/ast/ast.go

@@ -158,13 +158,24 @@ func MaxID(a *AST) int64 {
 func NewSourceInfo(src common.Source) *SourceInfo {
 	var lineOffsets []int32
 	var desc string
+	baseLine := int32(0)
+	baseCol := int32(0)
 	if src != nil {
 		desc = src.Description()
 		lineOffsets = src.LineOffsets()
+		// Determine whether the source metadata should be computed relative
+		// to a base line and column value. This can be determined by requesting
+		// the location for offset 0 from the source object.
+		if loc, found := src.OffsetLocation(0); found {
+			baseLine = int32(loc.Line()) - 1
+			baseCol = int32(loc.Column())
+		}
 	}
 	return &SourceInfo{
 		desc:         desc,
 		lines:        lineOffsets,
+		baseLine:     baseLine,
+		baseCol:      baseCol,
 		offsetRanges: make(map[int64]OffsetRange),
 		macroCalls:   make(map[int64]Expr),
 	}
@@ -200,6 +211,8 @@ type SourceInfo struct {
 	syntax       string
 	desc         string
 	lines        []int32
+	baseLine     int32
+	baseCol      int32
 	offsetRanges map[int64]OffsetRange
 	macroCalls   map[int64]Expr
 }
@@ -332,6 +345,10 @@ func (s *SourceInfo) GetStopLocation(id int64) common.Location {
 
 // ComputeOffset calculates the 0-based character offset from a 1-based line and 0-based column.
 func (s *SourceInfo) ComputeOffset(line, col int32) int32 {
+	if s != nil {
+		line = s.baseLine + line
+		col = s.baseCol + col
+	}
 	if line == 1 {
 		return col
 	}

common/ast/ast_test.go

@@ -294,3 +294,55 @@ func TestReferenceInfoAddOverload(t *testing.T) {
 		t.Error("repeated AddOverload() did not produce equal references")
 	}
 }
+
+func TestNewSourceInfoRelative(t *testing.T) {
+	sourceInfo := ast.NewSourceInfo(
+		mockRelativeSource(t,
+			"\n \n a || b ?\n cond1 :\n cond2",
+			[]int32{1, 2, 13, 25},
+			common.NewLocation(2, 1)))
+	tests := []struct {
+		loc    common.Location
+		offset int32
+	}{
+		// All locations specify a line number starting at 1
+		// The location of line 2, offset 1 is the same as the
+		// relative offset at location 1, 0 (offset 2)
+		{loc: common.NewLocation(1, 0), offset: 2},
+		// Equivalent to line 3, column 4
+		{loc: common.NewLocation(2, 3), offset: 6},
+		// Equivalent to line 4, column 2
+		{loc: common.NewLocation(3, 1), offset: 15},
+	}
+	for _, tst := range tests {
+		gotOffset := sourceInfo.ComputeOffset(int32(tst.loc.Line()), int32(tst.loc.Column()))
+		if gotOffset != tst.offset {
+			t.Errorf("ComputeOffset() got %v, wanted %v", gotOffset, tst.offset)
+		}
+	}
+}
+
+func mockRelativeSource(t testing.TB, text string, lineOffsets []int32, baseLocation common.Location) common.Source {
+	t.Helper()
+	return &mockSource{
+		Source:       common.NewTextSource(text),
+		lineOffsets:  lineOffsets,
+		baseLocation: baseLocation}
+}
+
+type mockSource struct {
+	common.Source
+	lineOffsets  []int32
+	baseLocation common.Location
+}
+
+func (src *mockSource) LineOffsets() []int32 {
+	return src.lineOffsets
+}
+
+func (src *mockSource) OffsetLocation(offset int32) (common.Location, bool) {
+	if offset == 0 {
+		return src.baseLocation, true
+	}
+	return src.Source.OffsetLocation(offset)
+}