3 files changed, 126 insertions, 0 deletions
diff --git a/snap/parsing.go b/snap/parsing.go
new file mode 100644
index 0000000..a848c33
--- /dev/null
+++ b/snap/parsing.go
@@ -0,0 +1,89 @@
+package snap
+
+import (
+	"strings"
+	"log"
+	"os"
+	"strconv"
+)
+
+var die = log.New(os.Stderr, "", 0)
+
+func isEscaped(str string) bool {
+	n := len(str)
+	return n > 1 && str[0] == '"' && str[n - 1] == '"'; 
+}
+
+func parseEscaped(snapish string) *Relative {
+	if isEscaped(snapish) {
+		snapshot := strings.Trim(snapish, `"`);
+
+		if(snapshot == "") {
+			die.Fatal("Snapshot cannot be empty \"\"")
+		
+		}
+
+		return &Relative {
+			snapshot: snapshot,
+		}
+	}
+
+	return nil
+}
+
+func parseRelativeSyntax(snapish, directionToken string) *Relative {
+	parts := strings.Split(snapish, directionToken)
+	n_parts := len(parts)
+
+	if n_parts == 1 {
+		return nil;
+	}
+	
+	// Test if the last the last token is a numeric offset
+	if offset, err := strconv.Atoi(parts[n_parts - 1]); 
+		err == nil {
+		
+		// Two tokens (e.g. ++ or --) indicates increment syntax
+		if n_parts > 2 && parts[n_parts - 2] == "" {
+			die.Fatal("A snapshot reference cannot be a mix of increment syntax (e.g. my_snap++) and literal syntax (e.g. my_snap+4).")
+		}
+
+		snapshot := strings.TrimRight(snapish, directionToken + "0123456789")
+		
+		return &Relative {
+			offset: offset,
+			snapshot: snapshot,
+		}
+	}
+
+	// Parse the increment / decrement syntax
+	// E.g. snapshot+++ or snapshot---
+	offset := 0;
+	for i := n_parts - 1; i >= 1 && parts[i] == ""; i-- {
+		offset += 1
+	}
+  
+	return &Relative {
+		offset: offset,
+		snapshot: snapish[:len(snapish) - offset],
+	}
+}
+
+func ToRelative(snapish string) * Relative {
+	if ref := parseEscaped(snapish); ref != nil {
+		return ref;
+	}
+
+	if ref := parseRelativeSyntax(snapish, "+"); ref != nil {
+		return ref;
+	}
+
+	if ref := parseRelativeSyntax(snapish, "-"); ref != nil {
+		ref.offset = -ref.offset;
+		return ref;
+	}
+
+	return &Relative{
+		snapshot: snapish,
+	};
+}
+\ No newline at end of file
diff --git a/snap/parsing_test.go b/snap/parsing_test.go
new file mode 100644
index 0000000..1a87723
--- /dev/null
+++ b/snap/parsing_test.go
@@ -0,0 +1,31 @@
+package snap
+
+import "testing"
+
+func TestRelativeParsing(t * testing.T) {
+	cases := []struct {
+		snapish string
+		snapshot string
+		offset int
+	} {
+		{"snapshot", "snapshot", 0},
+		{"testing--", "testing", -2},
+		{"%SNAPSHOT%^+++", "%SNAPSHOT%^", 3},
+		{"--prefixed", "--prefixed", 0},
+		{"+++", "", 3},
+		{"---", "", -3},
+		{"+5", "", 5},
+		{"-3", "", -3},
+		{"+", "", 1},
+		{"-", "", -1},
+		{`"-"`, "-", 0},
+	}
+
+	for _, c := range cases {
+		got := ToRelative(c.snapish)
+
+		if got.offset != c.offset || got.snapshot != c.snapshot {
+			t.Errorf("ToRelative(%s) == %+v, wanted %+v", c.snapish, got, c)
+		}
+	}
+}
+\ No newline at end of file
diff --git a/snap/snap.go b/snap/snap.go
new file mode 100644
index 0000000..fde60c4
--- /dev/null
+++ b/snap/snap.go
@@ -0,0 +1,6 @@
+package snap
+
+type Relative struct {
+	snapshot string
+	offset   int
+}
+\ No newline at end of file