op-challenger: Fault Proof Game Interfaces (#6052)

op-challenger/fault/main.go

+package main
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/ethereum/go-ethereum/common"
+)
+
+// Position is a golang wrapper around the dispute game Position type.
+// Depth refers to how many bisection steps have occurred.
+// IndexAtDepth refers to the path that the bisection has taken
+// where 1 = goes right & 0 = goes left.
+type Position struct {
+	Depth        int
+	IndexAtDepth int
+}
+
+// TraceIndex calculates the what the index of the claim value
+// would be inside the trace.
+func (p *Position) TraceIndex(maxDepth int) int {
+	lo := 0
+	hi := 1 << maxDepth
+	mid := hi
+	path := p.IndexAtDepth
+	for i := p.Depth - 1; i >= 0; i-- {
+		mid = (lo + hi) / 2
+		mask := 1 << i
+		if path&mask == mask {
+			lo = mid
+		} else {
+			hi = mid
+		}
+	}
+	return mid
+}
+
+func (p *Position) move(right bool) {
+	p.Depth++
+	p.IndexAtDepth = (p.IndexAtDepth << 1) | boolToInt(right)
+}
+
+func boolToInt(b bool) int {
+	if b {
+		return 1
+	} else {
+		return 0
+	}
+}
+
+func (p *Position) parent() {
+	p.Depth--
+	p.IndexAtDepth = p.IndexAtDepth >> 1
+}
+
+func (p *Position) Attack() {
+	p.move(false)
+}
+
+func (p *Position) Defend() {
+	p.parent()
+	p.move(true)
+	p.move(false)
+}
+
+func (p *Position) Print(maxDepth int) {
+	fmt.Printf("Trace Position is %04b\tTrace Depth is: %d\tTrace Index is: %d\n", p.IndexAtDepth, p.Depth, p.TraceIndex(maxDepth))
+}
+
+var (
+	ErrNegativeIndex = errors.New("index cannot be negative")
+	ErrIndexTooLarge = errors.New("index is larger than the maximum index")
+)
+
+// TraceProvider is a generic way to get a claim value at a specific
+// step in the trace.
+type TraceProvider interface {
+	Get(i int) (common.Hash, error)
+}
+
+type Claim struct {
+	Value common.Hash
+	Position
+}
+
+type Response struct {
+	Attack bool // note: can we flip this to true == going right / defending??
+	Value  common.Hash
+}
+
+func main() {
+	// Example 1
+	// abcdefgh
+	// abcdexyz
+	// go left to d, then right to f, then left to e
+	p := Position{}
+	p.Print(3)
+	p.Attack()
+	p.Print(3)
+	p.Defend()
+	p.Print(3)
+	p.Attack()
+	p.Print(3)
+
+	// Trace Position is 0000	Trace Depth is: 0	Trace Index is: 8
+	// Trace Position is 0000	Trace Depth is: 1	Trace Index is: 4
+	// Trace Position is 0010	Trace Depth is: 2	Trace Index is: 6
+	// Trace Position is 0100	Trace Depth is: 3	Trace Index is: 5
+
+	// Example 2
+	// abcdefgh
+	// abqrstuv
+	// go left r, then left to b, then right to q
+	p = Position{}
+	p.Print(3)
+	p.Attack()
+	p.Print(3)
+	p.Attack()
+	p.Print(3)
+	p.Defend()
+	p.Print(3)
+
+	// Trace Position is 0000	Trace Depth is: 0	Trace Index is: 8
+	// Trace Position is 0000	Trace Depth is: 1	Trace Index is: 4
+	// Trace Position is 0000	Trace Depth is: 2	Trace Index is: 2
+	// Trace Position is 0010	Trace Depth is: 3	Trace Index is: 3
+}