feat(filters): Show similar filter names when missing filter(#186)

Sources/Parser.swift

@@ -88,7 +88,26 @@ public class TokenParser {
       }
     }
 
-    throw TemplateSyntaxError("Unknown filter '\(name)'")
+    let suggestedFilters = self.suggestedFilters(for: name)
+    if suggestedFilters.isEmpty {
+      throw TemplateSyntaxError("Unknown filter '\(name)'.")
+    } else {
+      throw TemplateSyntaxError("Unknown filter '\(name)'. Found similar filters: \(suggestedFilters.map({ "'\($0)'" }).joined(separator: ", "))")
+    }
+  }
+
+  private func suggestedFilters(for name: String) -> [String] {
+    let allFilters = environment.extensions.flatMap({ $0.filters.keys })
+
+    let filtersWithDistance = allFilters
+                .map({ (filterName: $0, distance: $0.levenshteinDistance(name)) })
+                // do not suggest filters which names are shorter than the distance
+                .filter({ $0.filterName.characters.count > $0.distance })
+    guard let minDistance = filtersWithDistance.min(by: { $0.distance < $1.distance })?.distance else {
+      return []
+    }
+    // suggest all filters with the same distance
+    return filtersWithDistance.filter({ $0.distance == minDistance }).map({ $0.filterName })
   }
 
   public func compileFilter(_ token: String) throws -> Resolvable {
@@ -96,3 +115,45 @@ public class TokenParser {
   }
 
 }
+
+// https://en.wikipedia.org/wiki/Levenshtein_distance#Iterative_with_two_matrix_rows
+extension String {
+
+  subscript(_ i: Int) -> Character {
+    return self[self.index(self.startIndex, offsetBy: i)]
+  }
+
+  func levenshteinDistance(_ target: String) -> Int {
+    // create two work vectors of integer distances
+    var last, current: [Int]
+
+    // initialize v0 (the previous row of distances)
+    // this row is A[0][i]: edit distance for an empty s
+    // the distance is just the number of characters to delete from t
+    last = [Int](0...target.characters.count)
+    current = [Int](repeating: 0, count: target.characters.count + 1)
+
+    for i in 0..<self.characters.count {
+      // calculate v1 (current row distances) from the previous row v0
+
+      // first element of v1 is A[i+1][0]
+      //   edit distance is delete (i+1) chars from s to match empty t
+      current[0] = i + 1
+
+      // use formula to fill in the rest of the row
+      for j in 0..<target.characters.count {
+        current[j+1] = Swift.min(
+          last[j+1] + 1,
+          current[j] + 1,
+          last[j] + (self[i] == target[j] ? 0 : 1)
+        )
+      }
+
+      // copy v1 (current row) to v0 (previous row) for next iteration
+      last = current
+    }
+
+    return current[target.characters.count]
+  }
+
+}