Algorithms and Data Structures

N.Wirth. Algorithms and Data Structures. Oberon version 185
FOR i := a.m TO R+1 BY -1 DO a.e[i] := a.e[i-1] END;
a.e[R] := u; INC(a.m)
ELSE (*overflow; split a into a, b and assign the middle entry to v*)
NEW(b);
IF R < N THEN (*insert in left pate a*)
v := a.e[N-1];
FOR i := N-1 TO R+1 BY -1 DO a.e[i] := a.e[i-1] END;
a.e[R] := u;
FOR i := 0 TO N-1 DO b.e[i] := a.e[i+N] END
ELSE (*insert in right page b*)
DEC(R, N);
IF R = 0 THEN
v := u
ELSE
v := a.e[N];
FOR i := 0 TO R-2 DO b.e[i] := a.e[i+N+1] END;
b.e[R-1] := u
END;
FOR i := R TO N-1 DO b.e[i] := a.e[i+N] END
END;
a.m := N; b.m := N; b.p0 := v.p; v.p := b
END
END
END
END
END insert;
PROCEDURE underflow (c, a: Page; s: INTEGER; VAR h: BOOLEAN);
(*a = underflow page, c = ancestor page,
s = index of deleted entry in c*)
VAR b: Page; i, k: INTEGER;
BEGIN
(*h & (a.m = N-1) & (c.e[s-1].p = a) *)
IF s < c.m THEN (*b := page to the right of a*)
b := c.e[s].p;
k := (b.m-N+1) DIV 2; (*k = nof items available on page b*)
a.e[N-1] := c.e[s]; a.e[N-1].p := b.p0;
IF k > 0 THEN (*balance by moving k-1 items from b to a*)
FOR i := 0 TO k-2 DO a.e[i+N] := b.e[i] END;
c.e[s] := b.e[k-1]; b.p0 := c.e[s].p;
c.e[s].p := b; DEC(b.m, k);
FOR i := 0 TO b.m-1 DO b.e[i] := b.e[i+k] END;
a.m := N-1+k; h := FALSE
ELSE (*merge pages a and b, discard b*)
FOR i := 0 TO N-1 DO a.e[i+N] := b.e[i] END;
DEC(c.m);
FOR i := s TO c.m-1 DO c.e[i] := c.e[i+1] END;
a.m := 2*N; h := c.m < N
END
ELSE (*b := page to the left of a*)
DEC(s);
IF s = 0 THEN b := c.p0 ELSE b := c.e[s-1].p END;