index bloat query understand

I got a query to calculate index bloat as follows:

http://pgsql.tapoueh.org/site/html/news/20080131.bloat.html

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CREATE OR REPLACE VIEW bloat AS
SELECT
schemaname, tablename, reltuples::bigint, relpages::bigint, otta,
ROUND(CASE WHEN otta=0 THEN 0.0 ELSE sml.relpages/otta::numeric
END,1) AS tbloat,
relpages::bigint - otta AS wastedpages,
bs*(sml.relpages-otta)::bigint AS wastedbytes,
pg_size_pretty((bs*(relpages-otta))::bigint) AS wastedsize,
iname, ituples::bigint, ipages::bigint, iotta,
ROUND(CASE WHEN iotta=0 OR ipages=0 THEN 0.0 ELSE
ipages/iotta::numeric END,1) AS ibloat,
CASE WHEN ipages < iotta THEN 0 ELSE ipages::bigint - iotta END AS
wastedipages,
CASE WHEN ipages < iotta THEN 0 ELSE bs*(ipages-iotta) END AS
wastedibytes,
CASE WHEN ipages < iotta THEN pg_size_pretty(0) ELSE
pg_size_pretty((bs*(ipages-iotta))::bigint) END AS wastedisize
FROM (
SELECT
schemaname, tablename, cc.reltuples, cc.relpages, bs,
CEIL((cc.reltuples*((datahdr+ma-
(CASE WHEN datahdr%ma=0 THEN ma ELSE datahdr%ma
END))+nullhdr2+4))/(bs-20::float)) AS otta,
COALESCE(c2.relname,'?') AS iname, COALESCE(c2.reltuples,0) AS
ituples, COALESCE(c2.relpages,0) AS ipages,
COALESCE(CEIL((c2.reltuples*(datahdr-12))/(bs-20::float)),0) AS
iotta -- very rough approximation, assumes all cols
FROM (
SELECT
ma,bs,schemaname,tablename,
(datawidth+(hdr+ma-(case when hdr%ma=0 THEN ma ELSE hdr%ma
END)))::numeric AS datahdr,
(maxfracsum*(nullhdr+ma-(case when nullhdr%ma=0 THEN ma ELSE
nullhdr%ma END))) AS nullhdr2
FROM (
SELECT
schemaname, tablename, hdr, ma, bs,
SUM((1-null_frac)*avg_width) AS datawidth,
MAX(null_frac) AS maxfracsum,
hdr+(
SELECT 1+count(*)/8
FROM pg_stats s2
WHERE null_frac<>0 AND s2.schemaname = s.schemaname AND
s2.tablename = s.tablename
) AS nullhdr
FROM pg_stats s, (
SELECT
(SELECT current_setting('block_size')::numeric) AS bs,
CASE WHEN substring(v,12,3) IN ('8.0','8.1','8.2') THEN 27
ELSE 23 END AS hdr,
CASE WHEN v ~ 'mingw32' THEN 8 ELSE 4 END AS ma
FROM (SELECT version() AS v) AS foo
) AS constants
GROUP BY 1,2,3,4,5
) AS foo
) AS rs
JOIN pg_class cc ON cc.relname = rs.tablename
JOIN pg_namespace nn ON cc.relnamespace = nn.oid AND nn.nspname =
rs.schemaname
LEFT JOIN pg_index i ON indrelid = cc.oid
LEFT JOIN pg_class c2 ON c2.oid = i.indexrelid
) AS sml
WHERE sml.relpages - otta > 0 OR ipages - iotta > 10
ORDER BY wastedbytes DESC, wastedibytes DESC;

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Could anyone please describe me what are the meaning of the followings in
the query:

# ma - CASE WHEN v ~ 'mingw32' THEN 8 ELSE 4 END AS ma
# CASE WHEN substring(v,12,3) IN ('8.0','8.1','8.2') THEN 27 ELSE 23 END AS
hdr
# COALESCE(CEIL((c2.reltuples*(datahdr-12))/(bs-20::float)),0) AS iotta
# (CASE WHEN datahdr%ma=0 THEN ma ELSE datahdr%ma
END))+nullhdr2+4))/(bs-20::float)) AS otta