feat: Enhance use of "old" existing message

docs/manual/conventional-commits.md

@@ -21,12 +21,12 @@ Here is the full syntax for a commit message:
 [optional footer]
 ```
 
-The standad says that description is meant to start with a _lowercase letter_, so this is applied throughout this project.
+The standard says that description is meant to start with a _lowercase letter_, so this is applied throughout this project.
 
 This project focuses only on the first part:
 
 ```
-<TYPE>(SCOPE): MESSAGE
+<TYPE>(SCOPE): FILE_CHANGE_DESCRIPTION
 ```
 
 

src/prepareCommitMsg.ts

@@ -4,6 +4,9 @@
  * This module ties together logic from modules in the `generate` module. So it is best kept outside
  * that.
  *
+ * The "message" is the full commit message. The "file change description" is the description
+ * portion, which describes how the files changed.
+ *
  * This module doesn't interact with the git CLI or the extension. It just deals with text.
  */
 import { lookupDiffIndexAction } from "./generate/action";
@@ -13,6 +16,36 @@ import { parseDiffIndex } from "./git/parseOutput";
 import { CONVENTIONAL_TYPE } from "./lib/constants";
 import { equal } from "./lib/utils";
 
+/**
+ * Join two strings together with a space, or just one string if only one is set.
+ *
+ * Trimming on the outside is necessary, in case only one item is set.
+ * Don't both trimming the items before joining them, as this project doesn't need that for where
+ * this is used.
+ */
+export function _cleanJoin(first: string, second: string) {
+  return [first, second].join(" ").trim();
+}
+
+/**
+ * Separate a message into a Conventional Commit prefix, if any, and the description.
+ *
+ * Require a colon to exist to detect prefix. i.e. 'ci' will be considered a description, but 'ci:'
+ * will be considered a prefix. This keeps the check simpler as we don't have to match against every
+ * type and we don' have to check if we are part of a word e.g. 'circus'.
+ */
+export function _splitMsg(msg: string) {
+  const [prefix, fileChangeDesc] = msg.includes(":")
+    ? msg.split(":")
+    : ["", msg];
+
+  const [customPrefix, typePrefix] = prefix.includes(" ")
+    ? prefix.split(" ", 2)
+    : ["", prefix];
+
+  return { customPrefix, typePrefix, fileChangeDesc: fileChangeDesc.trim() };
+}
+
 /**
  * Determine what the prefix should be for a file change, using Conventional Commit standard.
  */
@@ -31,9 +64,9 @@ export function _msgOne(line: string) {
   // Don't unpack as {x, y, from, to}
   // const fileChanges = parseDiffIndex(line)
   const prefix = _prefixFromChanges(line),
-    fileChangeMsg = oneChange(line);
+    fileChangeDesc = oneChange(line);
 
-  return { prefix, fileChangeMsg };
+  return { prefix, fileChangeDesc };
 }
 
 /**
@@ -67,7 +100,7 @@ export function _msgMulti(lines: string[]) {
   const conventions = lines.map(_prefixFromChanges);
   const convention = collapse(conventions);
 
-  return { prefix: convention, fileChangeMsg: namedFiles(lines) };
+  return { prefix: convention, fileChangeDesc: namedFiles(lines) };
 }
 
 /**
@@ -87,42 +120,64 @@ export function _msgFromChanges(diffIndexLines: string[]) {
 /**
  * Output a readable conventional commit message.
  */
-export function _formatMsg(prefix: CONVENTIONAL_TYPE, fileChangeMsg: string) {
+export function _formatMsg(prefix: CONVENTIONAL_TYPE, fileChangeDesc: string) {
   if (prefix === CONVENTIONAL_TYPE.UNKNOWN) {
-    return fileChangeMsg;
+    return fileChangeDesc;
   }
-  return `${prefix}: ${fileChangeMsg}`;
+  return `${prefix}: ${fileChangeDesc}`;
 }
 
 /**
  * Generate a new commit message and format it as a string.
  */
 export function _newMsg(lines: string[]) {
-  const { prefix, fileChangeMsg } = _msgFromChanges(lines);
+  const { prefix, fileChangeDesc } = _msgFromChanges(lines);
 
-  return _formatMsg(prefix, fileChangeMsg);
+  return _formatMsg(prefix, fileChangeDesc);
 }
 
 /**
  * Format commit message using old and new messages.
  *
- * For now, assume old message is a commit message template prefix and can always go in front,
- * removing any existing twice on either side for flexibility.
- *
- * Dev note - must make sure prefix and fileChangeMsg come in separately here, not as a combined
- * message.
+ * The point is to always use the new message and prefix, but respect the old message. If there is
+ * no new prefix type set, use the old one, and if that is not set then just a simple message will
+ * do.
  *
- * TODO: Check if the old message is already a PREFIX form or a PREFIX FILECHANGE form. This changes
- * the new message form.
+ * @param prefix The Conventional Commit prefix to use, as generated by the extension based on
+ * changed files.
+ * @param fileChangeDesc A description of changes, also generated.
+ * @param oldMsg What exists in the commit message box when the extension is run, whether typed or
+ * generated previously.
  */
 export function _combineOldAndNew(
   prefix: CONVENTIONAL_TYPE,
-  fileChangeMsg: string,
+  fileChangeDesc: string,
   oldMsg?: string
 ) {
-  const newMsg = _formatMsg(prefix, fileChangeMsg);
+  if (!oldMsg) {
+    return _formatMsg(prefix, fileChangeDesc);
+  }
+
+  const {
+    customPrefix: oldCustomPrefix,
+    typePrefix: oldTypePrefix,
+    fileChangeDesc: oldFileChangeDesc,
+  } = _splitMsg(oldMsg);
+
+  const fileChangeDescResult = _cleanJoin(oldFileChangeDesc, fileChangeDesc);
+
+  if (prefix !== CONVENTIONAL_TYPE.UNKNOWN) {
+    return `${_cleanJoin(oldCustomPrefix, prefix)}: ${fileChangeDescResult}`;
+  }
+
+  if (oldTypePrefix) {
+    return `${_cleanJoin(
+      oldCustomPrefix,
+      oldTypePrefix
+    )}: ${fileChangeDescResult}`;
+  }
 
-  return oldMsg ? `${oldMsg.trim()} ${newMsg}` : newMsg;
+  return fileChangeDescResult;
 }
 
 /**
@@ -137,9 +192,9 @@ function generateMsgWithOld(fileChanges: string[], oldMsg: string) {
       "Either `oldMsg` must not be empty, or use `generateNewMsg` instead."
     );
   }
-  const { prefix, fileChangeMsg } = _msgFromChanges(fileChanges);
+  const { prefix, fileChangeDesc } = _msgFromChanges(fileChanges);
 
-  return _combineOldAndNew(prefix, fileChangeMsg, oldMsg);
+  return _combineOldAndNew(prefix, fileChangeDesc, oldMsg);
 }
 
 /**