From ce1f446ac7d01227964acd0eba9d5085dac0533e Mon Sep 17 00:00:00 2001
From: Victor Lin <13424970+victorlin@users.noreply.github.com>
Date: Thu, 21 Nov 2024 11:33:50 -0800
Subject: [PATCH] Add type-safe function for n % 3

At first, I disabled @typescript-eslint/consistent-type-assertions for
the type assertion in the original code, but that ended up polluting the
_frame() function with details unrelated to the direct purpose of the
function. Extracting the modulus operation into a separate function and
adding the details there felt like a better scope.
---
 src/util/entropyCreateStateFromJsons.ts | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/src/util/entropyCreateStateFromJsons.ts b/src/util/entropyCreateStateFromJsons.ts
index f9d43e416..fddd5d01b 100644
--- a/src/util/entropyCreateStateFromJsons.ts
+++ b/src/util/entropyCreateStateFromJsons.ts
@@ -332,9 +332,24 @@ function _frame(
   genomeLength: number,
   positiveStrand: boolean,
 ): Frame {
-  return (positiveStrand ?
-    (start+phase-1)%3 :
-    Math.abs((end-phase-genomeLength)%3)) as Frame;
+  return positiveStrand ?
+    _mod3(start+phase-1) :
+    _mod3(end-phase-genomeLength);
+}
+
+/**
+ * Type-safe modulo operation. Return value is unsigned (i.e. non-negative).
+ */
+function _mod3(n: number): 0 | 1 | 2 {
+  if (!Number.isInteger(n)) {
+    throw new Error(`${n} is not an integer.`);
+  }
+
+  /* TypeScript cannot infer the exact range of values from a modulo operation,
+   * so it is manually provided.
+   */
+  // eslint-disable-next-line @typescript-eslint/consistent-type-assertions
+  return (Math.abs(n) % 3) as 0 | 1 | 2
 }
 
 /**