support String#[]= with 3 args

mrblib/string.rb

@@ -154,13 +154,48 @@ class String
   end
 
   ##
-  # Modify +self+ by replacing the content of +self+
-  # at the position +pos+ with +value+.
-  def []=(pos, value)
-    pos += self.length if pos < 0
-    b = self[0, pos]
-    a = self[pos + 1..-1]
-    self.replace([b, value, a].join(''))
+  # Modify +self+ by replacing the content of +self+.
+  # The portion of the string affected is determined using the same criteria as +String#[]+.
+  def []=(*args)
+    anum = args.size
+    if anum == 2
+      pos, value = args
+      if pos.kind_of? String
+        posnum = self.index(pos)
+        if posnum
+          b = self[0, posnum.to_i]
+          a = self[(posnum + pos.length)..-1]
+          self.replace([b, value, a].join(''))
+          return value
+        else
+          raise IndexError, "string not matched"
+        end
+      else
+        pos += self.length if pos < 0
+        if pos < 0 || pos > self.length
+          raise IndexError, "index #{args[0]} out of string"
+        end
+        b = self[0, pos.to_i]
+        a = self[pos + 1..-1]
+        self.replace([b, value, a].join(''))
+        return value
+      end
+    elsif anum == 3
+      pos, len, value = args
+      pos += self.length if pos < 0
+      if pos < 0 || pos > self.length
+        raise IndexError, "index #{args[0]} out of string"
+      end
+      if len < 0
+        raise IndexError, "negative length #{len}"
+      end
+      b = self[0, pos.to_i]
+      a = self[pos + len..-1]
+      self.replace([b, value, a].join(''))
+      return value
+    else
+      raise ArgumentError, "wrong number of arguments (#{anum} for 2..3)"
+    end
   end
 
   ##

test/t/string.rb

@@ -122,6 +122,69 @@ assert('String#[] with Range') do
   assert_equal 'bc', j2
 end
 
+assert('String#[]=') do
+  # length of args is 1
+  a = 'abc'
+  a[0] = 'X'
+  assert_equal 'Xbc', a
+
+  b = 'abc'
+  b[-1] = 'X'
+  assert_equal 'abX', b
+
+  c = 'abc'
+  assert_raise(IndexError) do
+    c[10] = 'X'
+  end
+
+  d = 'abc'
+  assert_raise(IndexError) do
+    d[-10] = 'X'
+  end
+
+  e = 'abc'
+  e[1.1] = 'X'
+  assert_equal 'aXc', e
+
+
+  # length of args is 2
+  a1 = 'abc'
+  assert_raise(IndexError) do
+    a1[0, -1] = 'X'
+  end
+
+  b1 = 'abc'
+  assert_raise(IndexError) do
+    b1[10, 0] = 'X'
+  end
+
+  c1 = 'abc'
+  assert_raise(IndexError) do
+    c1[-10, 0] = 'X'
+  end
+
+  d1 = 'abc'
+  d1[0, 0] = 'X'
+  assert_equal 'Xabc', d1
+
+  e1 = 'abc'
+  e1[1, 3] = 'X'
+  assert_equal 'aX', e1
+
+  # args is RegExp
+  # It will be tested in mrbgems.
+
+  # args is String
+  a3 = 'abc'
+  a3['bc'] = 'X'
+  assert_equal a3, 'aX'
+
+  b3 = 'abc'
+  assert_raise(IndexError) do
+    b3['XX'] = 'Y'
+  end
+end
+
 assert('String#capitalize', '15.2.10.5.7') do
   a = 'abc'
   a.capitalize