Circular buffer of log lines

Scenario

A logging agent on a server hands log lines to a shipper through a buffer with a fixed number of slots. The agent writes lines at the back. The shipper reads them from the front, in the order they were written.

When the buffer is full, a write is refused, so the agent knows to wait. When a line is read, its slot is free again. Memory use must stay fixed, so the buffer cannot grow.

Your task

Write run_log_buffer(capacity, commands, values).

capacity is the number of slots. The buffer starts empty.
commands is a list of words: write, read, front, rear, empty or full.
values[i] belongs to commands[i]. It is the line number to write for a write, and it is ignored for the other commands.
write adds a line at the back, if there is a free slot.
read removes the line at the front.
front and rear read the line at the front and at the back, without removing it.
empty asks whether the buffer holds no lines. full asks whether every slot is used.
Return one answer for every command, in order.
For write and read, the answer is 1 when it worked and 0 when it did not. A write to a full buffer fails, and so does a read from an empty one.
For front and rear, the answer is the line number, or -1 when the buffer is empty.
For empty and full, the answer is 1 for yes and 0 for no.

Examples

Example 1 (traced)#

text2 lines

Input:  capacity = 2, commands = ["write", "write", "write", "full", "read", "write", "rear"], values = [10, 20, 30, 0, 0, 40, 0]
Output: [1, 1, 0, 1, 1, 1, 40]

Step	Command	Lines in the buffer (front to back)	Answer
1	`write 10`	`10`	1
2	`write 20`	`10` `20`	1
3	`write 30`	`10` `20` (no free slot)	0
4	`full`	`10` `20`	1
5	`read`	`20`	1
6	`write 40`	`20` `40` (slot 0 is reused)	1
7	`rear`	`20` `40`	40

The third write is refused because both slots are used. After a read frees a slot, the write of 40 works, and 40 is at the back.

Example 2#

text2 lines

Input:  capacity = 1, commands = ["read", "empty", "front"], values = [0, 0, 0]
Output: [0, 1, -1]

Reading from an empty buffer fails.

Example 3#

text2 lines

Input:  capacity = 3, commands = ["write", "front", "rear", "full"], values = [5, 0, 0, 0]
Output: [1, 5, 5, 0]

With one line, the front and the rear are the same line.

Constraints

1 ≤ capacity ≤ 10^5
1 ≤ length of commands ≤ 10^5
commands has the same length as values
each command is write, read, front, rear, empty or full
0 ≤ values[i] ≤ 10^6

What does n ≤ 10^5 tell you?

A buffer that shifts every line forward after a read costs up to capacity steps for each read. With a capacity of 50,000 and 100,000 commands, that is billions of steps, which is too slow.

So no line may move. Keep a head index and a count, and let the indexes wrap round the fixed block.

Scenario

When the buffer is full, a write is refused, so the agent knows to wait. When a line is read, its slot is free again. Memory use must stay fixed, so the buffer cannot grow.

Your task

Write run_log_buffer(capacity, commands, values).

capacity is the number of slots. The buffer starts empty.
commands is a list of words: write, read, front, rear, empty or full.
values[i] belongs to commands[i]. It is the line number to write for a write, and it is ignored for the other commands.
write adds a line at the back, if there is a free slot.
read removes the line at the front.
front and rear read the line at the front and at the back, without removing it.
empty asks whether the buffer holds no lines. full asks whether every slot is used.
Return one answer for every command, in order.
For write and read, the answer is 1 when it worked and 0 when it did not. A write to a full buffer fails, and so does a read from an empty one.
For front and rear, the answer is the line number, or -1 when the buffer is empty.
For empty and full, the answer is 1 for yes and 0 for no.

Examples

Example 1 (traced)#

text2 lines

Input:  capacity = 2, commands = ["write", "write", "write", "full", "read", "write", "rear"], values = [10, 20, 30, 0, 0, 40, 0]
Output: [1, 1, 0, 1, 1, 1, 40]

Step	Command	Lines in the buffer (front to back)	Answer
1	`write 10`	`10`	1
2	`write 20`	`10` `20`	1
3	`write 30`	`10` `20` (no free slot)	0
4	`full`	`10` `20`	1
5	`read`	`20`	1
6	`write 40`	`20` `40` (slot 0 is reused)	1
7	`rear`	`20` `40`	40

The third write is refused because both slots are used. After a read frees a slot, the write of 40 works, and 40 is at the back.

Example 2#

text2 lines

Input:  capacity = 1, commands = ["read", "empty", "front"], values = [0, 0, 0]
Output: [0, 1, -1]

Reading from an empty buffer fails.

Example 3#

text2 lines

Input:  capacity = 3, commands = ["write", "front", "rear", "full"], values = [5, 0, 0, 0]
Output: [1, 5, 5, 0]

With one line, the front and the rear are the same line.

Constraints

1 ≤ capacity ≤ 10^5
1 ≤ length of commands ≤ 10^5
commands has the same length as values
each command is write, read, front, rear, empty or full
0 ≤ values[i] ≤ 10^6

What does n ≤ 10^5 tell you?

A buffer that shifts every line forward after a read costs up to capacity steps for each read. With a capacity of 50,000 and 100,000 commands, that is billions of steps, which is too slow.

So no line may move. Keep a head index and a count, and let the indexes wrap round the fixed block.