Episode 2
Code execution trace
Code line, i/j pointer movement, lps fallback, and return i-j.
Algorithm lab prototype
KMP: why i does not move back
This lesson uses a browser-generated execution trace to connect code branches, pointer movement, boundary conditions, and the final return value in one repeatable walkthrough.
Current episode
Interactive prototype
Step through code, memory, stack frame, and CPU stage without loading a video file.
HTML interactive trace
KMP execution generated in the browser
Step through the same trace without loading a video file. The code line, memory cells, CPU stage, and stack frame are all rendered from structured data.
kmp.go
Code Trace
buildLPS folded
Complete strStr function. The highlighted row is the current program counter.
1
func strStr(haystack, needle string) int {// entry2
n := len(haystack)// text length3
m := len(needle)// pattern length4
if m == 0 { return 0 }// empty pattern5
if n < m { return -1 }// impossible6
lps := buildLPS(needle)// pattern memory7
PC8
i, j := 0, 0// two pointers9
for i < n {// i only moves right10
if haystack[i] == needle[j] {// current chars equal11
i++12
j++// advance together13
} else if j > 0 {// some prefix matched14
j = lps[j-1]// i does not move15
} else {16
i++// no memory17
}18
if j == m { return i-j }// return start19
}20
return -1// not found21
}Memory
haystack
i
a
0
b
1
a
2
b
3
c
4
a
5
b
6
c
7
a
8
c
9
b
10
a
11
b
12
needle
j
a
0
b
1
c
2
a
3
c
4
lps
0
0
0
1
0
2
1
3
0
4
Abstract CPU
LOAD
BRANCH
WRITE
compare-
branchprepare loop
Call Stack
strStr frame
FP 0x7ff0
SP 0x7fb8
addrslotvalue
0x7ff8ctrlreturn addrcaller+pc
0x7ff0ctrlsaved FP0x8010
0x7fe8paramshaystack*heap[H]
0x7fe0paramsneedle*heap[N]
0x7fd8localsn13
0x7fd0localsm5
0x7fc8localslps*heap[L]
0x7fc0localsi0
0x7fb8localsj0
read only
Initialize stack frame
The abstract machine starts by placing i, j, n, and m in the stack frame.
Step 1 of 1010%
Video fallback
The rendered MP4 versions stay available while the HTML player is being evaluated.
Episode 3
Abstract machine trace
Program counter, stack frame, memory reads, branch decision, and write-back.
Boundary cases
Why empty needle returns 0, and why longer needle returns -1 before KMP starts.
buildLPS state
How i, length, and lps change when the pattern compares with itself.
Matching branches
Why j falls back through lps while haystack's i stays where it is.
How this fits into 5tldr
Step 1
Keep generated videos as versioned static assets under public/learning.
Step 2
Use one topic page per concept so each video has transcript, checkpoints, and follow-up practice.
Step 3
Start noindex for prototypes, then link from Resources or Learning Plans after the format works.