text: #include <stdlib.h>
  #include <string.h>
  #include "heatshrink_decoder.h"
  
  /* States for the polling state machine. */
  typedef enum {
      HSDS_TAG_BIT,               /* tag bit */
      HSDS_YIELD_LITERAL,         /* ready to yield literal byte */
      HSDS_BACKREF_INDEX_MSB,     /* most significant byte of index */
      HSDS_BACKREF_INDEX_LSB,     /* least significant byte of index */
      HSDS_BACKREF_COUNT_MSB,     /* most significant byte of count */
      HSDS_BACKREF_COUNT_LSB,     /* least significant byte of count */
      HSDS_YIELD_BACKREF,         /* ready to yield back-reference */
  } HSD_state;
  
  #if HEATSHRINK_DEBUGGING_LOGS
  #include <stdio.h>
  #include <ctype.h>
  #include <assert.h>
  #define LOG(...) fprintf(stderr, __VA_ARGS__)
  #define ASSERT(X) assert(X)
  static const char *state_names[] = {
      "tag_bit",
      "yield_literal",
      "backref_index_msb",
      "backref_index_lsb",
      "backref_count_msb",
      "backref_count_lsb",
      "yield_backref",
  };
  #else
  #define LOG(...) /* no-op */
  #define ASSERT(X) /* no-op */
  #endif
  
  typedef struct {
      uint8_t *buf;               /* output buffer */
      size_t buf_size;            /* buffer size */
      size_t *output_size;        /* bytes pushed to buffer, so far */
  } output_info;
  
  #define NO_BITS ((uint16_t)-1)
  
  /* Forward references. */
  static uint16_t get_bits(heatshrink_decoder *hsd, uint8_t count);
  static void push_byte(heatshrink_decoder *hsd, output_info *oi, uint8_t byte);
  
  #if HEATSHRINK_DYNAMIC_ALLOC
  heatshrink_decoder *heatshrink_decoder_alloc(uint16_t input_buffer_size,
                                               uint8_t window_sz2,
                                               uint8_t lookahead_sz2) {
      if ((window_sz2 < HEATSHRINK_MIN_WINDOW_BITS) ||
          (window_sz2 > HEATSHRINK_MAX_WINDOW_BITS) ||
          (input_buffer_size == 0) ||
          (lookahead_sz2 < HEATSHRINK_MIN_LOOKAHEAD_BITS) ||
          (lookahead_sz2 >= window_sz2)) {
          return NULL;
      }
      size_t buffers_sz = (1 << window_sz2) + input_buffer_size;
      size_t sz = sizeof(heatshrink_decoder) + buffers_sz;
      heatshrink_decoder *hsd = HEATSHRINK_MALLOC(sz);
      if (hsd == NULL) { return NULL; }
      hsd->input_buffer_size = input_buffer_size;
      hsd->window_sz2 = window_sz2;
      hsd->lookahead_sz2 = lookahead_sz2;
      heatshrink_decoder_reset(hsd);
      LOG("-- allocated decoder with buffer size of %zu (%zu + %u + %u)\n",
          sz, sizeof(heatshrink_decoder), (1 << window_sz2), input_buffer_size);
      return hsd;
  }
  
  void heatshrink_decoder_free(heatshrink_decoder *hsd) {
      size_t buffers_sz = (1 << hsd->window_sz2) + hsd->input_buffer_size;
      size_t sz = sizeof(heatshrink_decoder) + buffers_sz;
      HEATSHRINK_FREE(hsd, sz);
      (void)sz;   /* may not be used by free */
  }
  #endif
  
  void heatshrink_decoder_reset(heatshrink_decoder *hsd) {
      size_t buf_sz = 1 << HEATSHRINK_DECODER_WINDOW_BITS(hsd);
      size_t input_sz = HEATSHRINK_DECODER_INPUT_BUFFER_SIZE(hsd);
      memset(hsd->buffers, 0, buf_sz + input_sz);
      hsd->state = HSDS_TAG_BIT;
      hsd->input_size = 0;
      hsd->input_index = 0;
      hsd->bit_index = 0x00;
      hsd->current_byte = 0x00;
      hsd->output_count = 0;
      hsd->output_index = 0;
      hsd->head_index = 0;
  }
  
  /* Copy SIZE bytes into the decoder's input buffer, if it will fit. */
  HSD_sink_res heatshrink_decoder_sink(heatshrink_decoder *hsd,
          uint8_t *in_buf, size_t size, size_t *input_size) {
      if ((hsd == NULL) || (in_buf == NULL) || (input_size == NULL)) {
          return HSDR_SINK_ERROR_NULL;
      }
  
      size_t rem = HEATSHRINK_DECODER_INPUT_BUFFER_SIZE(hsd) - hsd->input_size;
      if (rem == 0) {
          *input_size = 0;
          return HSDR_SINK_FULL;
      }
  
      size = rem < size ? rem : size;
      LOG("-- sinking %zd bytes\n", size);
      /* copy into input buffer (at head of buffers) */
      memcpy(&hsd->buffers[hsd->input_size], in_buf, size);
      hsd->input_size += size;
      *input_size = size;
      return HSDR_SINK_OK;
  }
  
  
  /*****************
   * Decompression *
   *****************/
  
  #define BACKREF_COUNT_BITS(HSD) (HEATSHRINK_DECODER_LOOKAHEAD_BITS(HSD))
  #define BACKREF_INDEX_BITS(HSD) (HEATSHRINK_DECODER_WINDOW_BITS(HSD))
  
  // States
  static HSD_state st_tag_bit(heatshrink_decoder *hsd);
  static HSD_state st_yield_literal(heatshrink_decoder *hsd,
      output_info *oi);
  static HSD_state st_backref_index_msb(heatshrink_decoder *hsd);
  static HSD_state st_backref_index_lsb(heatshrink_decoder *hsd);
  static HSD_state st_backref_count_msb(heatshrink_decoder *hsd);
  static HSD_state st_backref_count_lsb(heatshrink_decoder *hsd);
  static HSD_state st_yield_backref(heatshrink_decoder *hsd,
      output_info *oi);
  
  HSD_poll_res heatshrink_decoder_poll(heatshrink_decoder *hsd,
          uint8_t *out_buf, size_t out_buf_size, size_t *output_size) {
      if ((hsd == NULL) || (out_buf == NULL) || (output_size == NULL)) {
          return HSDR_POLL_ERROR_NULL;
      }
      *output_size = 0;
  
      output_info oi;
      oi.buf = out_buf;
      oi.buf_size = out_buf_size;
      oi.output_size = output_size;
  
      while (1) {
          LOG("-- poll, state is %d (%s), input_size %d\n",
              hsd->state, state_names[hsd->state], hsd->input_size);
          uint8_t in_state = hsd->state;
          switch (in_state) {
          case HSDS_TAG_BIT:
              hsd->state = st_tag_bit(hsd);
              break;
          case HSDS_YIELD_LITERAL:
              hsd->state = st_yield_literal(hsd, &oi);
              break;
          case HSDS_BACKREF_INDEX_MSB:
              hsd->state = st_backref_index_msb(hsd);
              break;
          case HSDS_BACKREF_INDEX_LSB:
              hsd->state = st_backref_index_lsb(hsd);
              break;
          case HSDS_BACKREF_COUNT_MSB:
              hsd->state = st_backref_count_msb(hsd);
              break;
          case HSDS_BACKREF_COUNT_LSB:
              hsd->state = st_backref_count_lsb(hsd);
              break;
          case HSDS_YIELD_BACKREF:
              hsd->state = st_yield_backref(hsd, &oi);
              break;
          default:
              return HSDR_POLL_ERROR_UNKNOWN;
          }
          
          /* If the current state cannot advance, check if input or output
           * buffer are exhausted. */
          if (hsd->state == in_state) {
              if (*output_size == out_buf_size) { return HSDR_POLL_MORE; }
              return HSDR_POLL_EMPTY;
          }
      }
  }
  
  static HSD_state st_tag_bit(heatshrink_decoder *hsd) {
      uint32_t bits = get_bits(hsd, 1);  // get tag bit
      if (bits == NO_BITS) {
          return HSDS_TAG_BIT;
      } else if (bits) {
          return HSDS_YIELD_LITERAL;
      } else if (HEATSHRINK_DECODER_WINDOW_BITS(hsd) > 8) {
          return HSDS_BACKREF_INDEX_MSB;
      } else {
          hsd->output_index = 0;
          return HSDS_BACKREF_INDEX_LSB;
      }
  }
  
  static HSD_state st_yield_literal(heatshrink_decoder *hsd,
          output_info *oi) {
      /* Emit a repeated section from the window buffer, and add it (again)
       * to the window buffer. (Note that the repetition can include
       * itself.)*/
      if (*oi->output_size < oi->buf_size) {
          uint16_t byte = get_bits(hsd, 8);
          if (byte == NO_BITS) { return HSDS_YIELD_LITERAL; } /* out of input */
          uint8_t *buf = &hsd->buffers[HEATSHRINK_DECODER_INPUT_BUFFER_SIZE(hsd)];
          uint16_t mask = (1 << HEATSHRINK_DECODER_WINDOW_BITS(hsd))  - 1;
          uint8_t c = byte & 0xFF;
          LOG("-- emitting literal byte 0x%02x ('%c')\n", c, isprint(c) ? c : '.');
          buf[hsd->head_index++ & mask] = c;
          push_byte(hsd, oi, c);
          return HSDS_TAG_BIT;
      } else {
          return HSDS_YIELD_LITERAL;
      }
  }
  
  static HSD_state st_backref_index_msb(heatshrink_decoder *hsd) {
      uint8_t bit_ct = BACKREF_INDEX_BITS(hsd);
      ASSERT(bit_ct > 8);
      uint16_t bits = get_bits(hsd, bit_ct - 8);
      LOG("-- backref index (msb), got 0x%04x (+1)\n", bits);
      if (bits == NO_BITS) { return HSDS_BACKREF_INDEX_MSB; }
      hsd->output_index = bits << 8;
      return HSDS_BACKREF_INDEX_LSB;
  }
  
  static HSD_state st_backref_index_lsb(heatshrink_decoder *hsd) {
      uint8_t bit_ct = BACKREF_INDEX_BITS(hsd);
      uint16_t bits = get_bits(hsd, bit_ct < 8 ? bit_ct : 8);
      LOG("-- backref index (lsb), got 0x%04x (+1)\n", bits);
      if (bits == NO_BITS) { return HSDS_BACKREF_INDEX_LSB; }
      hsd->output_index |= bits;
      hsd->output_index++;
      uint8_t br_bit_ct = BACKREF_COUNT_BITS(hsd);
      hsd->output_count = 0;
      return (br_bit_ct > 8) ? HSDS_BACKREF_COUNT_MSB : HSDS_BACKREF_COUNT_LSB;
  }
  
  static HSD_state st_backref_count_msb(heatshrink_decoder *hsd) {
      uint8_t br_bit_ct = BACKREF_COUNT_BITS(hsd);
      ASSERT(br_bit_ct > 8);
      uint16_t bits = get_bits(hsd, br_bit_ct - 8);
      LOG("-- backref count (msb), got 0x%04x (+1)\n", bits);
      if (bits == NO_BITS) { return HSDS_BACKREF_COUNT_MSB; }
      hsd->output_count = bits << 8;
      return HSDS_BACKREF_COUNT_LSB;
  }
  
  static HSD_state st_backref_count_lsb(heatshrink_decoder *hsd) {
      uint8_t br_bit_ct = BACKREF_COUNT_BITS(hsd);
      uint16_t bits = get_bits(hsd, br_bit_ct < 8 ? br_bit_ct : 8);
      LOG("-- backref count (lsb), got 0x%04x (+1)\n", bits);
      if (bits == NO_BITS) { return HSDS_BACKREF_COUNT_LSB; }
      hsd->output_count |= bits;
      hsd->output_count++;
      return HSDS_YIELD_BACKREF;
  }
  
  static HSD_state st_yield_backref(heatshrink_decoder *hsd,
          output_info *oi) {
      size_t count = oi->buf_size - *oi->output_size;
      if (count > 0) {
          size_t i = 0;
          if (hsd->output_count < count) count = hsd->output_count;
          uint8_t *buf = &hsd->buffers[HEATSHRINK_DECODER_INPUT_BUFFER_SIZE(hsd)];
          uint16_t mask = (1 << HEATSHRINK_DECODER_WINDOW_BITS(hsd)) - 1;
          uint16_t neg_offset = hsd->output_index;
          LOG("-- emitting %zu bytes from -%u bytes back\n", count, neg_offset);
          ASSERT(neg_offset <= mask + 1);
          ASSERT(count <= (size_t)(1 << BACKREF_COUNT_BITS(hsd)));
  
          for (i=0; i<count; i++) {
              uint8_t c = buf[(hsd->head_index - neg_offset) & mask];
              push_byte(hsd, oi, c);
              buf[hsd->head_index & mask] = c;
              hsd->head_index++;
              LOG("  -- ++ 0x%02x\n", c);
          }
          hsd->output_count -= count;
          if (hsd->output_count == 0) { return HSDS_TAG_BIT; }
      }
      return HSDS_YIELD_BACKREF;
  }
  
  /* Get the next COUNT bits from the input buffer, saving incremental progress.
   * Returns NO_BITS on end of input, or if more than 15 bits are requested. */
  static uint16_t get_bits(heatshrink_decoder *hsd, uint8_t count) {
      uint16_t accumulator = 0;
      int i = 0;
      if (count > 15) { return NO_BITS; }
      LOG("-- popping %u bit(s)\n", count);
  
      /* If we aren't able to get COUNT bits, suspend immediately, because we
       * don't track how many bits of COUNT we've accumulated before suspend. */
      if (hsd->input_size == 0) {
          if (hsd->bit_index < (1 << (count - 1))) { return NO_BITS; }
      }
  
      for (i = 0; i < count; i++) {
          if (hsd->bit_index == 0x00) {
              if (hsd->input_size == 0) {
                  LOG("  -- out of bits, suspending w/ accumulator of %u (0x%02x)\n",
                      accumulator, accumulator);
                  return NO_BITS;
              }
              hsd->current_byte = hsd->buffers[hsd->input_index++];
              LOG("  -- pulled byte 0x%02x\n", hsd->current_byte);
              if (hsd->input_index == hsd->input_size) {
                  hsd->input_index = 0; /* input is exhausted */
                  hsd->input_size = 0;
              }
              hsd->bit_index = 0x80;
          }
          accumulator <<= 1;
          if (hsd->current_byte & hsd->bit_index) {
              accumulator |= 0x01;
              if (0) {
                  LOG("  -- got 1, accumulator 0x%04x, bit_index 0x%02x\n",
                  accumulator, hsd->bit_index);
              }
          } else {
              if (0) {
                  LOG("  -- got 0, accumulator 0x%04x, bit_index 0x%02x\n",
                  accumulator, hsd->bit_index);
              }
          }
          hsd->bit_index >>= 1;
      }
  
      if (count > 1) { LOG("  -- accumulated %08x\n", accumulator); }
      return accumulator;
  }
  
  HSD_finish_res heatshrink_decoder_finish(heatshrink_decoder *hsd) {
      if (hsd == NULL) { return HSDR_FINISH_ERROR_NULL; }
      switch (hsd->state) {
      case HSDS_TAG_BIT:
          return hsd->input_size == 0 ? HSDR_FINISH_DONE : HSDR_FINISH_MORE;
  
      /* If we want to finish with no input, but are in these states, it's
       * because the 0-bit padding to the last byte looks like a backref
       * marker bit followed by all 0s for index and count bits. */
      case HSDS_BACKREF_INDEX_LSB:
      case HSDS_BACKREF_INDEX_MSB:
      case HSDS_BACKREF_COUNT_LSB:
      case HSDS_BACKREF_COUNT_MSB:
          return hsd->input_size == 0 ? HSDR_FINISH_DONE : HSDR_FINISH_MORE;
  
      /* If the output stream is padded with 0xFFs (possibly due to being in
       * flash memory), also explicitly check the input size rather than
       * uselessly returning MORE but yielding 0 bytes when polling. */
      case HSDS_YIELD_LITERAL:
          return hsd->input_size == 0 ? HSDR_FINISH_DONE : HSDR_FINISH_MORE;
  
      default:
          return HSDR_FINISH_MORE;
      }
  }
  
  static void push_byte(heatshrink_decoder *hsd, output_info *oi, uint8_t byte) {
      LOG(" -- pushing byte: 0x%02x ('%c')\n", byte, isprint(byte) ? byte : '.');
      oi->buf[(*oi->output_size)++] = byte;
      (void)hsd;
  }
  