How it works

This section shows different buffer corner cases and provides basic understanding how data are managed internally.

Different buffer corner cases

Let’s start with reference of abbreviations in picture:

• R represents Read pointer. Read on read/write operations. Modified on read operation only

• W represents Write pointer. Read on read/write operations. Modified on write operation only

• S represents Size of buffer. Used on all operations, never modified (atomic value)

  • Valid number of W and R pointers are between 0 and S - 1

• Buffer size is S = 8, thus valid number range for W and R pointers is 0 - 7.

  • R and W numbers overflow at S, thus valid range is always 0, 1, 2, 3, ..., S - 2, S - 1, 0, 1, 2, 3, ..., S - 2, S - 1, 0, ...

  • Example S = 4: 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, ...

• Maximal number of bytes buffer can hold is always S - 1, thus example buffer can hold up to 7 bytes

• R and W pointers always point to the next read/write operation

• When W == R, buffer is considered empty.

• When W == R - 1, buffer is considered full.

  • W == R - 1 is valid only if W and R overflow at buffer size S.

  • Always add S to calculated number and then use modulus S to get final value

Note

Example 1, add 2 numbers: 2 + 3 = (3 + 2 + S) % S = (3 + 2 + 4) % 4 = (5 + 4) % 4 = 1

Example 2, subtract 2 numbers: 2 - 3 = (2 - 3 + S) % S = (2 - 3 + 4) % 4 = (-1 + 4) % 4 = 3

Different buffer corner cases

Different image cases:

• Case A: Buffer is empty as W == R = 0 == 0

• Case B: Buffer holds W - R = 4 - 0 = 4 bytes as W > R

• Case C: Buffer is full as W == R - 1 or 7 == 0 - 1 or 7 = (0 - 1 + S) % S = (0 - 1 + 8) % 8 = (-1 + 8) % 8 = 7

  • R and W can hold S different values, from 0 to S - 1, that is modulus of S

  • Buffer holds W - R = 7 - 0 = 7 bytes as W > R

• Case D: Buffer holds S - (R - W) = 8 - (5 - 3) = 6 bytes as R > W

• Case E: Buffer is full as W == R - 1 (4 = 5 - 1) and holds S - (R - W) = 8 - (5 - 4) ) = 7 bytes