rewrite version resolution, add tests and screenshot

Makefile

@@ -1,6 +1,7 @@
 .PHONY: test
 test:
 	npx eslint --color --quiet *.js
+	node --trace-deprecation --throw-deprecation test.js
 
 .PHONY: publish
 publish:

package.json

@@ -40,6 +40,7 @@
   "devDependencies": {
     "eslint": "5.12.1",
     "eslint-config-silverwind": "2.0.14",
+    "execa": "^1.0.0",
     "ver": "3.0.1"
   }
 }

test.js

@@ -0,0 +1,50 @@
+"use strict";
+
+const assert = require("assert");
+const process = require("process");
+const execa = require("execa");
+
+function exit(err) {
+  if (err) {
+    console.info(err);
+  }
+  process.exit(err ? 1 : 0);
+}
+
+async function run(args) {
+  return JSON.parse(await execa.stdout("./updates.js", args.split(/\s+/)));
+}
+
+async function main() {
+  assert.deepStrictEqual(await run("-j -f test.json"), {
+    results: {
+      "gulp-sourcemaps": {
+        old: "2.0.0",
+        new: "2.6.4",
+        info: "https://github.com/floridoo/gulp-sourcemaps"
+      },
+      "prismjs": {
+        old: "1.0.0",
+        new: "1.15.0",
+        info: "https://github.com/LeaVerou/prism"
+      }
+    }
+  });
+
+  assert.deepStrictEqual(await run("-j -g -f test.json"), {
+    results: {
+      "gulp-sourcemaps": {
+        old: "2.0.0",
+        new: "2.6.4",
+        info: "https://github.com/floridoo/gulp-sourcemaps"
+      },
+      "prismjs": {
+        old: "1.0.0",
+        new: "9000.0.2",
+        info: "https://github.com/LeaVerou/prism"
+      }
+    }
+  });
+}
+
+main().then(exit).catch(exit);

test.json

@@ -0,0 +1,6 @@
+{
+  "dependencies": {
+    "gulp-sourcemaps": "2.0.0",
+    "prismjs": "1.0.0"
+  }
+}

updates.js

@@ -324,7 +324,6 @@ function updatePkg() {
   return newPkgStr;
 }
 
-// naive regex replace
 function updateRange(range, version) {
   return range.replace(/[0-9]+\.[0-9]+\.[0-9]+(-.+)?/g, version);
 }
@@ -338,9 +337,25 @@ function isValidSemverRange(range) {
   return valid;
 }
 
-function findNewVersion(data, opts) {
-  const versions = Object.keys(data.time).filter(version => semver.valid(version));
-  let tempVersion = semver.coerce(opts.range) || "0.0.0";
+function isVersionPrerelease(version) {
+  return semver.parse(version).prerelease.length;
+}
+
+function isRangePrerelease(range) {
+  // can not use semver.coerce here because it ignores prerelease tags
+  return /[0-9]+\.[0-9]+\.[0-9]+-.+/.test(range);
+}
+
+function rangeToVersion(range) {
+  try {
+    return semver.coerce(range).version;
+  } catch (err) {
+    return "0.0.0";
+  }
+}
+
+function findVersion(data, versions, opts) {
+  let tempVersion = rangeToVersion(opts.range);
   let tempDate = 0;
 
   for (const version of versions) {
@@ -353,7 +368,7 @@ function findNewVersion(data, opts) {
     }
 
     if (!opts.semvers.includes(diff)) continue;
-    if (diff === "prerelease" && !opts.usePre) continue;
+    if (isVersionPrerelease(parsed.version) && !opts.usePre) continue;
 
     if (opts.useGreatest) {
       if (semver.gte(parsed.version, tempVersion)) {
@@ -368,17 +383,42 @@ function findNewVersion(data, opts) {
     }
   }
 
-  // Special case for when pre-releases are tagged as latest. This ignores the
-  // --prerelease option, but it's how npm and other tools work so we copy
-  // their behaviour.
-  const latestTag = data["dist-tags"].latest;
-  if (!opts.useGreatest && latestTag !== tempVersion && semver.diff(tempVersion, latestTag) === "prerelease") {
-    tempVersion = latestTag;
-  }
-
   return tempVersion === "0.0.0" ? null : tempVersion;
 }
 
+function findNewVersion(data, opts) {
+  const versions = Object.keys(data.time).filter(version => semver.valid(version));
+  const version = findVersion(data, versions, opts);
+
+  if (opts.useGreatest) {
+    return version;
+  } else {
+    const latestTag = data["dist-tags"].latest;
+    const oldVersion = semver.coerce(opts.range);
+    const oldIsPre = isRangePrerelease(opts.range);
+    const newIsPre = isVersionPrerelease(version);
+    const isGreater = semver.gt(version, oldVersion);
+
+    // update to new prerelease
+    if (opts.usePre && newIsPre && isGreater) {
+      return version;
+    }
+
+    // update from prerelease to release
+    if (oldIsPre && !newIsPre && isGreater) {
+      return version;
+    }
+
+    // do not downgrade from prerelease to release
+    if (oldIsPre && !newIsPre && !isGreater) {
+      return oldVersion;
+    }
+
+    // in all other cases, return latest dist-tag
+    return latestTag;
+  }
+}
+
 function parseMixedArg(arg) {
   if (arg === "") {
     return true;