fix(markdown): correctly determine count of backticks in inline code (#6110)

CHANGELOG.unreleased.md

@@ -126,11 +126,37 @@ const v = /** @type{string} */ value;
 const v = /** @type{string} */ (value);
 ```
 
+### Markdown: correctly determine count of backticks in inline code ([#6110] by [@belochub])
+
+By the CommonMark spec, it is required to 'choose a string of `n` backtick characters as delimiters, where the code does not contain any strings of exactly `n` backtick characters.'
+
+This changes the method of finding the required count of backticks from using 2 backticks, when there is a backtick string of length 1 inside the inline code block, and using 1 backtick in all other cases, to finding a minimum length backtick string that can correctly be used as a delimiter.
+
+<!-- prettier-ignore -->
+````md
+<!-- Input -->
+``` 3 ``22`` `1` ```
+
+`` 2 ```123``` `1` ``
+
+<!-- Output (Prettier stable) -->
+` 3 ``22`` `1` `
+
+` 2 ```123``` `1` `
+
+<!-- Output (Prettier master) -->
+``` 3 ``22`` `1` ```
+
+`` 2 ```123``` `1` ``
+````
+
 [#5979]: https://github.com/prettier/prettier/pull/5979
 [#6115]: https://github.com/prettier/prettier/pull/6115
 [#6106]: https://github.com/prettier/prettier/pull/6106
 [#6116]: https://github.com/prettier/prettier/pull/6116
+[#6110]: https://github.com/prettier/prettier/pull/6110
 [@jridgewell]: https://github.com/jridgewell
 [@jwbay]: https://github.com/jwbay
 [@brainkim]: https://github.com/brainkim
 [@sosukesuzuki]: https://github.com/sosukesuzuki
+[@belochub]: https://github.com/belochub

src/common/util.js

@@ -578,6 +578,35 @@ function getMaxContinuousCount(str, target) {
   );
 }
 
+function getMinNotPresentContinuousCount(str, target) {
+  const matches = str.match(
+    new RegExp(`(${escapeStringRegexp(target)})+`, "g")
+  );
+
+  if (matches === null) {
+    return 0;
+  }
+
+  const countPresent = new Map();
+  let max = 0;
+
+  for (const match of matches) {
+    const count = match.length / target.length;
+    countPresent.set(count, true);
+    if (count > max) {
+      max = count;
+    }
+  }
+
+  for (let i = 1; i < max; i++) {
+    if (!countPresent.get(i)) {
+      return i;
+    }
+  }
+
+  return max + 1;
+}
+
 function getStringWidth(text) {
   if (!text) {
     return 0;
@@ -681,6 +710,7 @@ module.exports = {
   replaceEndOfLineWith,
   getStringWidth,
   getMaxContinuousCount,
+  getMinNotPresentContinuousCount,
   getPrecedence,
   shouldFlatten,
   isBitwiseOperator,

src/language-markdown/printer-markdown.js

@@ -130,8 +130,11 @@ function genericPrint(path, options, print) {
     case "delete":
       return concat(["~~", printChildren(path, options, print), "~~"]);
     case "inlineCode": {
-      const backtickCount = privateUtil.getMaxContinuousCount(node.value, "`");
-      const style = backtickCount === 1 ? "``" : "`";
+      const backtickCount = privateUtil.getMinNotPresentContinuousCount(
+        node.value,
+        "`"
+      );
+      const style = "`".repeat(backtickCount || 1);
       const gap = backtickCount ? " " : "";
       return concat([style, gap, node.value, gap, style]);
     }

tests/markdown_inlineCode/__snapshots__/jsfmt.spec.js.snap

@@ -17,6 +17,10 @@ proseWrap: "always"
 
 \`\` \`\`\`123\`\`\` \`\`
 
+\`\`\` 3 \`\`22\`\` \`1\` \`\`\`
+
+\`\` 2 \`\`\`123\`\`\` \`1\` \`\`
+
 =====================================output=====================================
 \`\` \`123\` \`\`
 
@@ -28,6 +32,10 @@ proseWrap: "always"
 
 \` \`\`\`123\`\`\` \`
 
+\`\`\` 3 \`\`22\`\` \`1\` \`\`\`
+
+\`\` 2 \`\`\`123\`\`\` \`1\` \`\`
+
 ================================================================================
 `;
 

tests/markdown_inlineCode/backtick.md

@@ -7,3 +7,7 @@
 ``34` ``
 
 `` ```123``` ``
+
+``` 3 ``22`` `1` ```
+
+`` 2 ```123``` `1` ``