Arrange blocks in bands of 128 lines

This is more similar to what the Brother driver does. This might
fix #52, #40, etc.

src/block.h

@@ -25,7 +25,6 @@
 class block {
  public:
   block(): line_bytes_(0) {
-    lines_.reserve(max_lines_per_block_);
   }
 
   bool empty() const {
@@ -40,8 +39,7 @@ class block {
   }
 
   bool line_fits(unsigned size) {
-    return lines_.size() != max_lines_per_block_
-      && line_bytes_ + size < max_block_size_;
+    return line_bytes_ + size < max_block_size_;
   }
 
   void flush(FILE *f) {
@@ -59,7 +57,6 @@ class block {
 
  private:
   static const unsigned max_block_size_ = 16350;
-  static const unsigned max_lines_per_block_ = 64;
 
   std::vector<std::vector<uint8_t>> lines_;
   int line_bytes_;

src/job.cc

@@ -109,14 +109,22 @@ void job::encode_page(const page_params &page_params,
 
   fputs("\033*b1030m", out_);
 
+  // XXX brother driver uses 128 lines per band
+  const int lines_per_band = 64;
+
   for (int i = 1; i < lines && nextline(line); ++i) {
-    std::vector<uint8_t> encoded = encode_line(line, reference);
-    if (block.line_fits(encoded.size())) {
-      block.add_line(std::move(encoded));
-    } else {
+    std::vector<uint8_t> encoded;
+    if (i % lines_per_band == 0) {
       block.flush(out_);
-      block.add_line(encode_line(line));
+      encoded = encode_line(line);
+    } else {
+      encoded = encode_line(line, reference);
+      if (!block.line_fits(encoded.size())) {
+        block.flush(out_);
+        encoded = encode_line(line);
+      }
     }
+    block.add_line(std::move(encoded));
     std::swap(line, reference);
   }
 

test/test_block.cc

@@ -37,16 +37,6 @@ const lest::test specification[] = {
     EXPECT(!b.empty());
   },
 
-  "A block can contain 64 lines",
-  [] {
-    block b;
-    for (int i = 0; i < 64; ++i) {
-      EXPECT(b.line_fits(1));
-      b.add_line(vec(1));
-    }
-    EXPECT(!b.line_fits(1));
-  },
-
   "A block has a size limit of about 16 kilobyte",
   [] {
     block b;