11.4.6 compact_string_chars

The purpose of this function is to compact all the data parts of the strings that are held in so-called string_chars_block, i.e. the strings that do not exceed a certain maximal length.

The procedure with which this is done is as follows. We are keeping two positions in the string_chars_blocks using two pointer/integer pairs, namely from_sb/from_pos and to_sb/to_pos. They stand for the actual positions, from where to where, to copy the actually handled string.

While going over all chained string_char_blocks and their held strings, staring at first_string_chars_block, both pointers are advanced and eventually a string is copied from from_sb to to_sb, depending on the status of the pointed at strings.

More precisely, we can distinguish between the following actions.

• The string at from_sb’s position could be marked as free, which is indicated by an invalid pointer to the pointer that should point back to the fixed size string object, and which is checked by FREE_STRUCT_P. In this case, the from_sb/from_pos is advanced to the next string, and nothing has to be copied.
• Also, if a string object itself is unmarked, nothing has to be copied. We likewise advance the from_sb/from_pos pair as described above.
• In all other cases, we have a marked string at hand. The string data must be moved from the from-position to the to-position. In case there is not enough space in the actual to_sb-block, we advance this pointer to the beginning of the next block before copying. In case the from and to positions are different, we perform the actual copying using the library function memmove.

After compacting, the pointer to the current string_chars_block, sitting in current_string_chars_block, is reset on the last block to which we moved a string, i.e. to_block, and all remaining blocks (we know that they just carry garbage) are explicitly xfreed.