Consolidate attestation actions (#346)

* consolidate attestation actions

Signed-off-by: Brian DeHamer <bdehamer@github.com>

* better errors

Signed-off-by: Brian DeHamer <bdehamer@github.com>

* Update src/sbom.ts

Co-authored-by: Austin Beattie <ajbeattie@github.com>

* clarify dedupe comment

Signed-off-by: Brian DeHamer <bdehamer@github.com>

---------

Signed-off-by: Brian DeHamer <bdehamer@github.com>
Co-authored-by: Austin Beattie <ajbeattie@github.com>

src/detect.ts

@@ -0,0 +1,45 @@
+export type AttestationType = 'provenance' | 'sbom' | 'custom'
+
+export type DetectionInputs = {
+  sbomPath: string
+  predicateType: string
+  predicate: string
+  predicatePath: string
+}
+
+export const detectAttestationType = (
+  inputs: DetectionInputs
+): AttestationType => {
+  const { sbomPath, predicateType, predicate, predicatePath } = inputs
+
+  // SBOM mode takes priority
+  if (sbomPath) {
+    return 'sbom'
+  }
+
+  // Custom mode when any predicate inputs are provided
+  if (predicateType || predicate || predicatePath) {
+    return 'custom'
+  }
+
+  // Default to provenance mode
+  return 'provenance'
+}
+
+export const validateAttestationInputs = (inputs: DetectionInputs): void => {
+  const { sbomPath, predicateType, predicate, predicatePath } = inputs
+
+  // Cannot combine sbom-path with predicate inputs
+  if (sbomPath && (predicateType || predicate || predicatePath)) {
+    throw new Error(
+      'Cannot specify sbom-path together with predicate-type, predicate, or predicate-path'
+    )
+  }
+
+  // Custom mode requires predicate-type
+  if ((predicate || predicatePath) && !predicateType) {
+    throw new Error(
+      'predicate-type is required when using predicate or predicate-path'
+    )
+  }
+}