VisiCalc Reconstructed

VisiCalc Reconstructed
zserge.com

  #define MAXIN 128  // max cell input length
  enum { EMPTY, NUM, LABEL, FORMULA };  // cell types
  struct cell {
    int type;
    float val;
    char text[MAXIN];  // raw user input
  };
  #define NCOL 26    // max number of columns (A..Z)
  #define NROW 50    // max number of rows
  struct grid {
    struct cell cells[NCOL][NROW];
  };

> The basic approach was to allocate memory into fixed chunks so that we wouldn't have a problem with the kind of breakage that occurs with irregular allocation. Deallocating a cell freed up 100% of its storage. Thus a given spreadsheet would take up the same amount of space no matter how it was created. I presumed that the spreadsheet would normally be compact and in the upper left (low number rows and cells) so used a vector of rows vectors. The chunks were also called cells so I had to be careful about terminology to avoid confusion. Internally the term "cell" always meant storage cell. These cells were allocated from one direction and the vectors from the other. When they collided the program reorganized the storage. It had to do this in place since there was no room left at that point -- after all that's why we had to do the reorganization.

> The actual representation was variable length with each element prefixed by a varying length type indicator. In order to avoid having most code parse the formula the last by was marked $ff (or 0xff in today's representation). It turned out that valid cell references at the edges of the sheet looked like this and created some interesting bugs.

It leaves out a lot of details - if you're skimping enough you could allocate variable length row vectors, but it seems they wanted to avoid variable length allocations, in which case you could start with a 255 byte array pointing to which subsequent equal-sized chunk represents each in-use row. You'd need at most 126 bytes per row in actual use to point into the chunks representing the cell contents. But this is just guesses.

[1] https://www.landley.net/history/mirror/apple2/implementingvi... and https://news.ycombinator.com/item?id=34303825