p2+p2len            -3-2-1
p2                  +0+1+2
p1+p1len-6-5-4-3-2-1
p1      +0+1+2+3+4+5
         0 1 2 3 4 5 6 7 8 
        [a b c d e f 1 2 3] temp                           I     RHS
                    [1]>>>>>>[1]       temp   = *(p2+0)    0      *(p2+0)
              [d]>>>[d]               *(p2+0) = *(p1+3)    0      *(p1+(p1len-p2len))
        [a]>>>[a]                     *(p1+3) = *(p1+0)    0      *(p1+I)
        [1]<<<<<<<<<<<<<<<<<<[1]      *(p1+0) =  temp
                      [2]>>>>[2]       temp   = *(p2+1)    1      *(p2+I)
                [e]>>>[e]             *(p2+1) = *(p1+4)    1      *(p1+(p1len-p2len)+I)
          [b]>>>[b]                   *(p1+4) = *(p1+1)    1      *(p1+I)
          [2]<<<<<<<<<<<<<<<<[2]      *(p1+1) =  temp
                        [3]>>[3]       temp   = *(p2+2)    2      *(p2+I)
                  [f]>>>[f]           *(p2+2) = *(p1+5)    2      *(p1+(p1len-p2len)+I)
            [c]>>>[c]                 *(p1+5) = *(p1+2)    2      *(p1+I)
            [3]<<<<<<<<<<<<<<[3]      *(p1+2) =  temp    
        [1 2 3 a b c d e f]   

##</code><code>##

p2+p2len                  -4-3-2-1
p2                        +0+1+2+3
p1+p1len-9-8-7-6-5-4-3-2-1                                       L1=last element of buffer 1
p1      +0+1+2+3+4+5+6+7+8                                       L2=last element of buffer 2
         0 1 2 3 4 5 6 7 8                                       diff=p1len-p2len=5
        [a b c d e f g h i 1 2 3 4] temp                         I
                                [4]>>[4]      temp   = *(p2+3)   0      *(L2+I)
                        [i]>>>>>[i]          *(p2+3) = *(p1+8)   0      *(L1+I)
                [e]>>>>>[e]                  *(p1+8) = *(p1+4)   0      *(L1-diff+1+I)
        [a]>>>>>[a]                          *(p1+4) = *(p1+0)   1      *(L2+I)
        [1]<<<<<<<<<<<<<<<[1]                *(p1+0) = *(p2+0)   1      *(L1+I)
                  [f]>>>>>[f]                *(p2+0) = *(p1+5)   1      *(L1-diff+1+I)
          [b]>>>>>[b]                        *(p1+5) = *(p1+1)   2      *(L2+I)     
          [2]<<<<<<<<<<<<<<<[2]              *(p1+1) = *(p2+1)   2      *(L1+I)   # I think you had a typo here 
                    [g]>>>>>[g]              *(p2+2) = *(p1+6)   2      *(L1-diff+1+I)
            [c]>>>>>[c]                      *(p1+6) = *(p1+2)   3      *(L2+I)
            [3]<<<<<<<<<<<<<<<[3]            *(p1+2) = *(p2+2)   3      *(L1+I)
                      [h]>>>>>[h]            *(p2+2) = *(p1+7)   3      *(L1-diff+1+I)
              [d]>>>>>[d]                    *(p1+7) = *(p1+3)   4      *(L2+I)
              [4]<<<<<<<<<<<<<<<<<<<<[4]     *(p1+3) =  temp
        [1 2 3 4 a b c d e f g h i]