Add DrawLine solution

Author: mdanilov

Diff for: src/05_bit_manipulation/draw_line.cpp

@@ -2,9 +2,68 @@
 // eight consecutive pixels to be stored in one byte. The screen has width w,
 // where w is divisible by 8 (that is, no byte will be split across rows). The
 // height of the screen, of course, can be derived from the length of the array
-// and t he width. Implement a function that draws a horizontal line from (x1,
+// and the width. Implement a function that draws a horizontal line from (x1,
 // y) to (x2, y).
 // The method signature should look something like:
 // drawLine(byte[] screen, int width, int x1, int x2, int y)
 //
 // Hints: #366, #381, #384, #391
+
+#include <cstdint>
+#include <vector>
+
+// Solution
+// ------------------------------------------------------------------------------------------------
+void drawLine(std::vector<uint8_t> &screen, int width, int x1, int x2, int y) {
+  int start_offset = x1 % 8;
+  int first_full_byte = x1 / 8;
+  if (start_offset != 0) {
+    first_full_byte++;
+  }
+
+  int end_offset = x2 % 8;
+  int last_full_byte = x2 / 8;
+  if (end_offset != 7) {
+    last_full_byte--;
+  }
+
+  // Set full bytes
+  for (int b = first_full_byte; b <= last_full_byte; b++) {
+    screen[(width / 8) * y + b] = 0xFFU;
+  }
+
+  // Create masks for start and end of line
+  uint8_t start_mask = (0xFFU >> start_offset);
+  uint8_t end_mask = ~(0xFFU >> (end_offset + 1));
+
+  // Set start and end of line
+  if ((x1 / 8) == (x2 / 8)) { // x1 and x2 are in the same byte
+    uint8_t mask = (start_mask & end_mask);
+    screen[(width / 8) * y + (x1 / 8)] |= mask;
+  } else {
+    if (start_offset != 0) {
+      int byte_number = (width / 8) * y + first_full_byte - 1;
+      screen[byte_number] |= start_mask;
+    }
+    if (end_offset != 7) {
+      int byte_number = (width / 8) * y + last_full_byte + 1;
+      screen[byte_number] |= end_mask;
+    }
+  }
+}
+
+// Test
+// ------------------------------------------------------------------------------------------------
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+TEST(DrawLineTest, Trivial) {
+  int width = 48;
+  int height = 1;
+  std::vector<uint8_t> screen((width / 8) * height, 0);
+  drawLine(screen, width, 12, 28, 0);
+  ASSERT_THAT(screen,
+              ::testing::ElementsAre(0b00000000, 0b00001111, 0b11111111,
+                                     0b11111000, 0b00000000, 0b00000000));
+}