Measure external fragmentation

typedef struct {
  int unsigned lo;
  int unsigned hi;
} range_t;

class RangeAllocator;
  range_t free_ranges[$];
  int unsigned managed_base;
  int unsigned managed_limit;

  function new(int unsigned base, int unsigned size);
    if (size == 0) $fatal(1, "allocator size must be positive");
    if (base > 32'hffff_ffff - (size - 1))
      $fatal(1, "managed range wraps the address space");
    managed_base = base;
    managed_limit = base + size - 1;
    free_ranges.push_back('{lo: managed_base, hi: managed_limit});
  endfunction

  function bit alloc(int unsigned size, output int unsigned addr);
    if (size == 0) $fatal(1, "allocation size must be positive");
    foreach (free_ranges[i]) begin
      longint unsigned available =
        longint'(free_ranges[i].hi) - free_ranges[i].lo + 1;
      if (available >= size) begin
        addr = free_ranges[i].lo;
        if (available == size) free_ranges.delete(i);
        else free_ranges[i].lo += size;
        return 1;
      end
    end
    return 0;
  endfunction

  function void dealloc(int unsigned addr, int unsigned size);
    if (size == 0) $fatal(1, "deallocation size must be positive");
    if (addr > 32'hffff_ffff - (size - 1))
      $fatal(1, "deallocated range wraps the address space");
    free_ranges.push_back('{lo: addr, hi: addr + size - 1});
    coalesce();
  endfunction

  function void coalesce();
    range_t merged[$];
    range_t current;
    if (free_ranges.size() <= 1) return;
    free_ranges.sort() with (item.lo);
    current = free_ranges[0];
    for (int i = 1; i < free_ranges.size(); i++) begin
      if (free_ranges[i].lo <= current.hi ||
          (current.hi != 32'hffff_ffff &&
           free_ranges[i].lo == current.hi + 1))
        current.hi = (free_ranges[i].hi > current.hi) ?
                     free_ranges[i].hi : current.hi;
      else begin
        merged.push_back(current);
        current = free_ranges[i];
      end
    end
    merged.push_back(current);
    free_ranges = merged;
  endfunction

  function void fragmentation_metrics(
    output longint unsigned total_free,
    output longint unsigned largest_block,
    output real external_ratio
  );
    // Implement here: fragmentation_metrics.
  endfunction
endclass