Add method for simplifying ranges

Need this for a case where we programmatically generate a list of all
the versions subject to a security vulnerability by virtue of depending
exclusively on vulnerable versions of a given dependency.

While it's not a perfect heuristic, with this method on semver, instead
of printing out something long and confusing like this:

    8.0.1 || 8.0.2 || 9.0.0 || 9.0.1 || 10.0.0-alpha.0 || 10.0.0-alpha.1
    || 10.0.0-alpha.2 || 10.0.0-alpha.3 || 10.0.0-alpha.4 || 10.0.0 ||
    10.0.1 || 10.0.2 || 10.0.3 || 10.1.0 || 10.1.1 || 10.1.2 || 11.0.0
    || 11.1.0 || 12.0.0-candidate.0 || 12.0.0 || 12.0.1

we can show this:

    8.0.1 - 11.1.0 || 12.0.0-candidate.0 - 12.0.1

README.md

@@ -78,6 +78,7 @@ const semverOutside = require('semver/ranges/outside')
 const semverGtr = require('semver/ranges/gtr')
 const semverLtr = require('semver/ranges/ltr')
 const semverIntersects = require('semver/ranges/intersects')
+const simplifyRange = require('semver/ranges/simplify')
 ```
 
 As a command-line utility:
@@ -446,6 +447,14 @@ strings that they parse.
   `hilo` argument must be either the string `'>'` or `'<'`.  (This is
   the function called by `gtr` and `ltr`.)
 * `intersects(range)`: Return true if any of the ranges comparators intersect
+* `simplifyRange(versions, range)`: Return a "simplified" range that
+  matches the same items in `versions` list as the range specified.  Note
+  that it does *not* guarantee that it would match the same versions in all
+  cases, only for the set of versions provided.  This is useful when
+  generating ranges by joining together multiple versions with `||`
+  programmatically, to provide the user with something a bit more
+  ergonomic.  If the provided range is shorter in string-length than the
+  generated range, then that is returned.
 
 Note that, since ranges may be non-contiguous, a version might not be
 greater than a range, less than a range, *or* satisfy a range!  For

index.js

@@ -43,4 +43,5 @@ module.exports = {
   gtr: require('./ranges/gtr'),
   ltr: require('./ranges/ltr'),
   intersects: require('./ranges/intersects'),
+  simplifyRange: require('./ranges/simplify'),
 }

ranges/simplify.js

@@ -0,0 +1,44 @@
+// given a set of versions and a range, create a "simplified" range
+// that includes the same versions that the original range does
+// If the original range is shorter than the simplified one, return that.
+const satisfies = require('../functions/satisfies.js')
+const compare = require('../functions/compare.js')
+module.exports = (versions, range, options) => {
+  const set = []
+  let min = null
+  let prev = null
+  const v = versions.sort((a, b) => compare(a, b, options))
+  for (const version of v) {
+    const included = satisfies(version, range, options)
+    if (included) {
+      prev = version
+      if (!min)
+        min = version
+    } else {
+      if (prev) {
+        set.push([min, prev])
+      }
+      prev = null
+      min = null
+    }
+  }
+  if (min)
+    set.push([min, null])
+
+  const ranges = []
+  for (const [min, max] of set) {
+    if (min === max)
+      ranges.push(min)
+    else if (!max && min === v[0])
+      ranges.push('*')
+    else if (!max)
+      ranges.push(`>=${min}`)
+    else if (min === v[0])
+      ranges.push(`<=${max}`)
+    else
+      ranges.push(`${min} - ${max}`)
+  }
+  const simplified = ranges.join(' || ')
+  const original = typeof range.raw === 'string' ? range.raw : String(range)
+  return simplified.length < original.length ? simplified : range
+}

test/ranges/simplify.js

@@ -0,0 +1,42 @@
+const simplify = require('../../ranges/simplify.js')
+const Range = require('../../classes/range.js')
+const t = require('tap')
+const versions = [
+  '1.0.0',
+  '1.0.1',
+  '1.0.2',
+  '1.0.3',
+  '1.0.4',
+  '1.1.0',
+  '1.1.1',
+  '1.1.2',
+  '1.2.0',
+  '1.2.1',
+  '1.2.2',
+  '1.2.3',
+  '1.2.4',
+  '1.2.5',
+  '2.0.0',
+  '2.0.1',
+  '2.1.0',
+  '2.1.1',
+  '2.1.2',
+  '2.2.0',
+  '2.2.1',
+  '2.2.2',
+  '2.3.0',
+  '2.3.1',
+  '2.4.0',
+  '3.0.0',
+  '3.1.0',
+  '3.2.0',
+  '3.3.0',
+]
+
+t.equal(simplify(versions, '1.x'), '1.x')
+t.equal(simplify(versions, '1.0.0 || 1.0.1 || 1.0.2 || 1.0.3 || 1.0.4'), '<=1.0.4')
+t.equal(simplify(versions, new Range('1.0.0 || 1.0.1 || 1.0.2 || 1.0.3 || 1.0.4')), '<=1.0.4')
+t.equal(simplify(versions, '>=3.0.0 <3.1.0'), '3.0.0')
+t.equal(simplify(versions, '3.0.0 || 3.1 || 3.2 || 3.3'), '>=3.0.0')
+t.equal(simplify(versions, '1 || 2 || 3'), '*')
+t.equal(simplify(versions, '2.1 || 2.2 || 2.3'), '2.1.0 - 2.3.1')