document, expose, and test 'partial:true' option

README.md

@@ -171,6 +171,22 @@ Suppress the behavior of treating a leading `!` character as negation.
 Returns from negate expressions the same as if they were not negated.
 (Ie, true on a hit, false on a miss.)
 
+### partial
+
+Compare a partial path to a pattern.  As long as the parts of the path that
+are present are not contradicted by the pattern, it will be treated as a
+match.  This is useful in applications where you're walking through a
+folder structure, and don't yet have the full path, but want to ensure that
+you do not walk down paths that can never be a match.
+
+For example,
+
+```js
+minimatch('/a/b', '/a/*/c/d', { partial: true })  // true, might be /a/b/c/d
+minimatch('/a/b', '/**/d', { partial: true })     // true, might be /a/b/.../d
+minimatch('/x/y/z', '/a/**/z', { partial: true }) // false, because x !== a
+```
+
 ## Comparisons to other fnmatch/glob implementations
 
 While strict compliance with the existing standards is a worthwhile

minimatch.js

@@ -146,6 +146,7 @@ function Minimatch (pattern, options) {
   this.negate = false
   this.comment = false
   this.empty = false
+  this.partial = !!options.partial
 
   // make the set of regexps etc.
   this.make()
@@ -719,13 +720,15 @@ minimatch.match = function (list, pattern, options) {
   return list
 }
 
-Minimatch.prototype.match = function match (f) {
+Minimatch.prototype.match = function match (f, partial = this.partial) {
   this.debug('match', f, this.pattern)
   // short-circuit in the case of busted things.
   // comments, etc.
   if (this.comment) return false
   if (this.empty) return f === ''
 
+  if (f === '/' && partial) return true
+
   var options = this.options
 
   // windows: need to use /, not \
@@ -759,7 +762,7 @@ Minimatch.prototype.match = function match (f) {
     if (options.matchBase && pattern.length === 1) {
       file = [filename]
     }
-    var hit = this.matchOne(file, pattern, false)
+    var hit = this.matchOne(file, pattern, partial)
     if (hit) {
       if (options.flipNegate) return true
       return !this.negate

test/partial.js

@@ -0,0 +1,5 @@
+const t = require('tap')
+const mm = require('../')
+t.equal(mm('/a/b', '/*/b/x/y/z', { partial: true }), true)
+t.equal(mm('/a/b/c', '/*/b/x/y/z', { partial: true }), false)
+t.equal(mm('/', 'x', { partial: true }), true)