fix: allow semver arguments to be reversed (#31)

these changes allow the static semver value provided to the `:semver`
pseudo selector to be placed in either the first or second parameter.
that information is then used when calling the `subset` semver function
to allow users to reverse the parameters.

lib/index.js

@@ -85,31 +85,57 @@ const fixupNestedPseudo = astNode => {
   transformAst(newRootNode)
 }
 
-// :semver(<version|range>, [selector], [function])
+// :semver(<version|range|selector>, [version|range|selector], [function])
+// note: the first or second parameter must be a static version or range
 const fixupSemverSpecs = astNode => {
-  // the first child node contains the version or range, most likely as a tag and a series of
-  // classes. we combine them into a single string here. this is the only required input.
-  const children = astNode.nodes.shift().nodes
-  const value = children.reduce((res, i) => `${res}${String(i)}`, '')
-
-  // next, if we have 2 nodes left then the user called us with a total of 3. that means the
-  // last one tells us what specific semver function the user is requesting, so we pull that out
-  let semverFunc
-  if (astNode.nodes.length === 2) {
+  // if we have three nodes, the last is the semver function to use, pull that out first
+  if (astNode.nodes.length === 3) {
     const funcNode = astNode.nodes.pop().nodes[0]
     if (funcNode.type === 'tag') {
-      semverFunc = funcNode.value
+      astNode.semverFunc = funcNode.value
+    } else if (funcNode.type === 'string') {
+      // a string is always in some type of quotes, we don't want those so slice them off
+      astNode.semverFunc = funcNode.value.slice(1, -1)
+    } else {
+      // anything that isn't a tag or a string isn't a function name
+      throw Object.assign(
+        new Error('`:semver` pseudo-class expects a function name as last value'),
+        { code: 'ESEMVERFUNC' }
+      )
+    }
+  }
+
+  // now if we have 1 node, it's a static value
+  // istanbul ignore else
+  if (astNode.nodes.length === 1) {
+    const semverNode = astNode.nodes.pop()
+    astNode.semverValue = semverNode.nodes.reduce((res, next) => `${res}${String(next)}`, '')
+  } else if (astNode.nodes.length === 2) {
+    // and if we have two nodes, one of them is a static value and we need to determine which it is
+    for (let i = 0; i < astNode.nodes.length; ++i) {
+      const type = astNode.nodes[i].nodes[0].type
+      // the type of the first child may be combinator for ranges, such as >14
+      if (type === 'tag' || type === 'combinator') {
+        const semverNode = astNode.nodes.splice(i, 1)[0]
+        astNode.semverValue = semverNode.nodes.reduce((res, next) => `${res}${String(next)}`, '')
+        astNode.semverPosition = i
+        break
+      }
+    }
+
+    if (typeof astNode.semverValue === 'undefined') {
+      throw Object.assign(
+        new Error('`:semver` pseudo-class expects a static value in the first or second position'),
+        { code: 'ESEMVERVALUE' }
+      )
     }
   }
 
-  // now if there's a node left, that node is our selector. since that is the last remaining
-  // child node, we call fixupAttr on ourselves so that the attribute selectors get parsed
+  // if we got here, the last remaining child should be attribute selector
   if (astNode.nodes.length === 1) {
     fixupAttr(astNode)
   } else {
-    // we weren't provided a selector, so we default to `[version]`. note, there's no default
-    // operator here. that's because we don't know yet if the user has provided us a version
-    // or range to assert against
+    // if we don't have a selector, we default to `[version]`
     astNode.attributeMatcher = {
       insensitive: false,
       attribute: 'version',
@@ -118,8 +144,6 @@ const fixupSemverSpecs = astNode => {
     astNode.lookupProperties = []
   }
 
-  astNode.semverFunc = semverFunc
-  astNode.semverValue = value
   astNode.nodes.length = 0
 }