From 6b57e56a3932d5e7bdef4e7254cad8c247e76b9e Mon Sep 17 00:00:00 2001
From: Mark Spanbroek <mark@spanbroek.net>
Date: Tue, 19 Mar 2024 09:39:32 +0100
Subject: [PATCH] abi decoding of simple custom errors

Only supports errors without arguments for now
---
 ethers/errors.nim                | 17 +++++++++++++++++
 testmodule/test.nim              |  1 +
 testmodule/testErrorDecoding.nim | 30 ++++++++++++++++++++++++++++++
 3 files changed, 48 insertions(+)
 create mode 100644 ethers/errors.nim
 create mode 100644 testmodule/testErrorDecoding.nim

diff --git a/ethers/errors.nim b/ethers/errors.nim
new file mode 100644
index 0000000..508e485
--- /dev/null
+++ b/ethers/errors.nim
@@ -0,0 +1,17 @@
+import pkg/contractabi/selector
+import ./basics
+
+type SolidityError* = object of EthersError
+
+{.push raises:[].}
+
+template errors*(types) {.pragma.}
+
+func decode*[E: SolidityError](_: type E, data: seq[byte]): ?!(ref E) =
+  const name = $E
+  const selector = selector(name, typeof(()))
+  if data.len < 4:
+    return failure "unable to decode " & name & ": signature too short"
+  if selector.toArray[0..<4] != data[0..<4]:
+    return failure "unable to decode " & name & ": signature doesn't match"
+  success (ref E)()
diff --git a/testmodule/test.nim b/testmodule/test.nim
index eb8b23a..854579b 100644
--- a/testmodule/test.nim
+++ b/testmodule/test.nim
@@ -7,5 +7,6 @@ import ./testWallet
 import ./testTesting
 import ./testErc20
 import ./testGasEstimation
+import ./testErrorDecoding
 
 {.warning[UnusedImport]:off.}
diff --git a/testmodule/testErrorDecoding.nim b/testmodule/testErrorDecoding.nim
new file mode 100644
index 0000000..fa1d67f
--- /dev/null
+++ b/testmodule/testErrorDecoding.nim
@@ -0,0 +1,30 @@
+import std/unittest
+import pkg/questionable/results
+import pkg/ethers/errors
+
+suite "Decoding of custom errors":
+
+  type SimpleError = object of SolidityError
+
+  test "decodes a simple error":
+    let decoded = SimpleError.decode(@[0xc2'u8, 0xbb, 0x94, 0x7c])
+    check decoded is ?!(ref SimpleError)
+    check decoded.isSuccess
+    check (!decoded) != nil
+
+  test "returns failure when decoding fails":
+    let invalid = @[0xc2'u8, 0xbb, 0x94, 0x0] # last byte is wrong
+    let decoded = SimpleError.decode(invalid)
+    check decoded.isFailure
+
+  test "returns failure when data is less than 4 bytes":
+    let invalid = @[0xc2'u8, 0xbb, 0x94]
+    let decoded = SimpleError.decode(invalid)
+    check decoded.isFailure
+
+  test "decoding only works for SolidityErrors":
+    type InvalidError = ref object of CatchableError
+    const works = compiles:
+      InvalidError.decode(@[0x1'u8, 0x2, 0x3, 0x4])
+    check not works
+