Fix totals for areas in comments

[sgomes]

GitHub comments by ICFY currently have an issue with calculating totals for a particular "area". For example, for sections:

**Sections** (~119368 bytes removed 📉 [gzipped])

name                   parsed_size            gzip_size
signup                   -159335 B  (-31.0%)   -37498 B  (-29.4%)
checkout                 -152990 B   (-9.3%)   -35377 B   (-8.7%)
gutenberg-editor         -149228 B  (-26.6%)   -35530 B  (-23.0%)
reader                     -2907 B   (-0.5%)     -508 B   (-0.3%)
plans                      -2625 B   (-0.3%)     -399 B   (-0.2%)
jetpack-connect            -2436 B   (-0.3%)     -471 B   (-0.3%)
jetpack-cloud-pricing      -2436 B   (-0.6%)     -471 B   (-0.4%)
hosting                    -2436 B   (-0.8%)     -471 B   (-0.6%)
woocommerce                -1107 B   (-0.0%)     -307 B   (-0.1%)
posts-custom                -677 B   (-0.2%)     -151 B   (-0.1%)
posts                       -677 B   (-0.2%)     -151 B   (-0.1%)
settings                    -540 B   (-0.1%)     -590 B   (-0.4%)
privacy                     -410 B   (-0.1%)    -1066 B   (-1.3%)
settings-security           -398 B   (-0.1%)      -34 B   (-0.0%)
settings-writing            -391 B   (-0.1%)      -64 B   (-0.0%)
plugins                     -380 B   (-0.1%)      -38 B   (-0.0%)
purchases                   -339 B   (-0.0%)     +113 B   (+0.0%)
site-blocks                 -314 B   (-0.1%)    -1037 B   (-1.2%)
account                     -303 B   (-0.1%)    -1038 B   (-1.0%)
media                       -273 B   (-0.1%)     +299 B   (+0.2%)
wp-super-cache              -247 B   (-0.1%)      -30 B   (-0.0%)
settings-performance        -247 B   (-0.1%)      -30 B   (-0.0%)
settings-discussion         -247 B   (-0.1%)      -30 B   (-0.0%)
marketing                   -247 B   (-0.0%)      -30 B   (-0.0%)
earn                        -247 B   (-0.1%)      -25 B   (-0.0%)
site-purchases              -234 B   (-0.0%)     +608 B   (+0.3%)
home                        -203 B   (-0.0%)     -347 B   (-0.3%)
stats                       -195 B   (-0.0%)      -49 B   (-0.0%)
google-my-business          -169 B   (-0.1%)      -24 B   (-0.0%)
security                    -163 B   (-0.0%)    -1031 B   (-0.8%)
notification-settings       -163 B   (-0.0%)    -1031 B   (-1.1%)
me                          -163 B   (-0.1%)    -1031 B   (-1.4%)
happychat                   -163 B   (-0.1%)    -1031 B   (-1.2%)
account-close               -163 B   (-0.0%)    -1031 B   (-1.1%)
zoninator                   -151 B   (-0.1%)      -10 B   (-0.0%)
people                      -142 B   (-0.0%)      -70 B   (-0.1%)
import                      -142 B   (-0.1%)     -119 B   (-0.2%)
domains                     -135 B   (-0.0%)     +145 B   (+0.1%)
pages                       +101 B   (+0.0%)     +142 B   (+0.2%)
devdocs                      -95 B   (-0.0%)       -7 B   (-0.0%)
help                         -52 B   (-0.0%)       -1 B   (-0.0%)
email                        +42 B   (+0.0%)     +453 B   (+0.5%)

We see that the total is being calculated simply by adding up all the individual diffs for each group. This is incorrect, since groups can share chunks, and if a shared chunk changes size, it will be counted multiple times with this approach.

This PR instead takes shared chunks into account, by adding up all the chunks in use across an entire area for the "before" and "after" commits, and subtracting those sums instead (note: the values are slightly different because the above is from a PR and the below is from its merge):

**Sections** (~54533 bytes removed 📉 [gzipped])

name                   parsed_size            gzip_size
signup                   -159335 B  (-31.0%)   -37497 B  (-29.3%)
checkout                 -152990 B   (-9.3%)   -35377 B   (-8.7%)
gutenberg-editor         -149228 B  (-26.6%)   -35525 B  (-23.0%)
reader                     -2907 B   (-0.5%)     -508 B   (-0.3%)
plans                      -2625 B   (-0.3%)     -399 B   (-0.2%)
jetpack-connect            -2436 B   (-0.3%)     -471 B   (-0.3%)
jetpack-cloud-pricing      -2436 B   (-0.6%)     -471 B   (-0.4%)
hosting                    -2436 B   (-0.8%)     -471 B   (-0.6%)
woocommerce                -1107 B   (-0.0%)     -305 B   (-0.1%)
posts-custom                -677 B   (-0.2%)     -151 B   (-0.1%)
posts                       -677 B   (-0.2%)     -151 B   (-0.1%)
settings                    -556 B   (-0.1%)     -606 B   (-0.4%)
privacy                     -410 B   (-0.1%)    -1066 B   (-1.3%)
settings-security           -398 B   (-0.1%)      -34 B   (-0.0%)
settings-writing            -391 B   (-0.1%)      -64 B   (-0.0%)
plugins                     -380 B   (-0.1%)      -38 B   (-0.0%)
purchases                   -339 B   (-0.0%)     +113 B   (+0.0%)
site-blocks                 -314 B   (-0.1%)    -1037 B   (-1.2%)
account                     -303 B   (-0.1%)    -1038 B   (-1.0%)
media                       -273 B   (-0.1%)     +299 B   (+0.2%)
wp-super-cache              -247 B   (-0.1%)      -30 B   (-0.0%)
settings-performance        -247 B   (-0.1%)     +106 B   (+0.1%)
settings-discussion         -247 B   (-0.1%)      -30 B   (-0.0%)
earn                        -247 B   (-0.1%)      -25 B   (-0.0%)
marketing                   -235 B   (-0.0%)     +362 B   (+0.3%)
site-purchases              -234 B   (-0.0%)     +608 B   (+0.3%)
people                      -219 B   (-0.1%)     -842 B   (-0.9%)
home                        -203 B   (-0.0%)     -347 B   (-0.3%)
stats                       -195 B   (-0.0%)      -49 B   (-0.0%)
security                    -163 B   (-0.0%)    -1031 B   (-0.8%)
notification-settings       -163 B   (-0.0%)    -1031 B   (-1.1%)
me                          -163 B   (-0.1%)    -1031 B   (-1.4%)
happychat                   -163 B   (-0.1%)    -1031 B   (-1.2%)
account-close               -163 B   (-0.0%)    -1031 B   (-1.1%)
zoninator                   -151 B   (-0.1%)      -10 B   (-0.0%)
import                      -142 B   (-0.1%)      -93 B   (-0.1%)
domains                     -135 B   (-0.0%)     +145 B   (+0.1%)
pages                       +101 B   (+0.0%)     +142 B   (+0.2%)
devdocs                      -95 B   (-0.0%)       -7 B   (-0.0%)
google-my-business           -80 B   (-0.0%)     +990 B   (+1.1%)
help                         -52 B   (-0.0%)       -1 B   (-0.0%)
email                        +42 B   (+0.0%)     +453 B   (+0.5%)

This should reduce confusion, and more accurately convey the impact of a PR.

[jsnajdr]

Looks good, just pointing out one suspicious place in the code 🙂

I pushed an extra commit that declares loop variables in for loops.

[jsnajdr]

Is it possible that the chunksInUse[ index ] array is empty for a particular area? Then the reduce would loop over an empty array and would return an empty object rather than { stat_size: 0, parsed_size: 0, gzip_size: 0 }.

It might be better to do the reduce with a { stat_size: 0, parsed_size: 0, gzip_size: 0 } initial value.

The delta.js module has a nice sumSizesOf helper to sum two size objects together.

[sgomes]

I pushed an extra commit that declares loop variables in for loops.

Oops, good catch! I need to figure out a way of having my IDE warn me about those regardless of the project ESLint config.