Before looking at TeX itself, you may want to look at a few smaller programs.

The program below, GLUE, has an interesting history. (Reproduced from my answer here.)

In the November 1981 issue of TUGboat, Knuth mentioned that he was developing a system called WEB. Then in the next (March 1982) issue, he published “Fixed-Point Glue Setting: An Example of WEB”, saying

I will soon be publishing a complete manual about WEB, but in the meantime I think it will be useful to have an example of a fairly short piece of code written in “web” form. Therefore I have prepared the accompanying program, which also serves another function: …

You can read the program itself either on this page, or with texdoc glue, or here. The source glue.web is here.

I was able to test this glue.web just now with another Pascal compiler (fpc), by first running tangle glue.web to generate glue.p, then adding {$mode ISO} to the top of glue.p (so that when the program says “integer”, a 32-bit type is used rather than a 16-bit type), then running fpc glue.p to generate the glue binary. (I tried web2js but it crashed.)

Then to test the binary, put the following input (which consists of 7 test cases—pairs of lines—followed by 0) in a file (or type it in at the terminal). This input is the same as that used by Knuth.

200000
30000 40000 50000 60000 0
2000
30000 40000 50000 60000 0
1000000000
8000000 -9000000 8000000 4000 7000000 0
100
8000000 -9000000 8000000 4000 7000000 0
1000000000
800 -900 800 400 700 0
1000000000
800 -900 800 400 -700 0
65555 
-200 199 0
1
60000 -59999 90000 0
0

The output should be (matching that published in TUGboat):

Test data set number 1:
  Glue ratio is 1.1111 (0,14,18205)
               30000          33334
               40000          44445
               50000          55557
               60000          66668
 Totals       180000         200004 (versus 200000)
Test data set number 2:
  Glue ratio is 0.0111 (0,21,23302)
               30000            333
               40000            444
               50000            555
               60000            666
 Totals       180000           1998 (versus 2000)
Test data set number 3:
  Glue ratio is 71.4101 (8,0,18281)
             8000000      571281250
            -9000000     -642686836
             8000000      571281250
                4000         274215
             7000000      499857383
 Totals     14004000     1000007262 (versus 1000000000)
Test data set number 4:
  Glue ratio is 0.0000 (8,24,30670)
             8000000             57
            -9000000            -64
             8000000             57
                4000              0
             7000000             49
 Totals     14004000             99 (versus 100)
Test data set number 5:
  Glue ratio is 2x2x2x2x2x2x8681.0000 (-6,1,17362)
                 800      444467200
                -900     -500025600
                 800      444467200
                 400      222233600
                 700      388908800
 Totals         1800     1000051200 (versus 1000000000)
Test data set number 6:
! Excessive glue.
  Glue ratio is 2x2x2x2x2x2x2x0.0000 (-6,0,0)
                 800              0
                -900              0
                 800              0
                 400              0
                -700              0
 Totals          400              0 (versus 1000000000)
Test data set number 7:
Invalid data (nonpositive sum); this set rejected.
Test data set number 8:
  Glue ratio is 0.0000 (1,30,23861)
               60000              0
              -59999              0
               90000              1
 Totals        90001              1 (versus 1)