fix: scientific notation with plain numbers

_extensions/units/units.lua

@@ -64,35 +64,66 @@ local function formatSci(value)
   return out
 end
 
+local function finalizePlainString(s)
+  local sign, intPart, rest = s:match("^(%-?)(%d+)(.*)$")
+  if intPart then
+    s = sign .. groupDigits(intPart) .. rest
+  end
+  return displayMinus(s)
+end
+
 -- render a value in plain decimal notation, 3 significant figures.
 -- IMPORTANT: C's %g spec has its own internal rule for switching to
 -- exponential form ("use %e if exponent < -4 or exponent >= precision").
 -- With precision 3, that means %.3g silently flips to "1e+03"-style
 -- output for anything >= 1000 -- BEFORE any magnitude threshold of ours
--- gets a say. Rather than duplicating that rule with a second threshold
+-- gets a say. Rather than duplicating that rule with a second threshold,
 -- we let %g decide, then intercept its exponential output and re-render it through
 -- formatSci() instead of ever emitting the raw "1e+03" string.
+-- NOTE: this function is used for the "auto" mode ONLY. It is allowed to
+-- fall back to scientific notation. For an explicit, non-negotiable
+-- notation="plain" override, see formatForcedPlain below instead.
 local function formatPlain(value)
   local s = string.format("%.3g", value)
   if s:find("[eE]") then
     return formatSci(value)
   end
-  -- split into optional sign, integer part, and remainder (decimal point
-  -- + fractional digits, if any) so grouping applies only to the
-  -- integer part -- fractional digits are never grouped
-  local sign, intPart, rest = s:match("^(%-?)(%d+)(.*)$")
-  if intPart then
-    s = sign .. groupDigits(intPart) .. rest
+  return { pandoc.Str(finalizePlainString(s)) }
+end
+
+-- render a value in plain decimal notation that NEVER falls back to
+-- scientific form, regardless of magnitude, this is what
+-- notation="plain" actually promises the caller. %g's own exponent-switch
+-- rule (the reason formatPlain above sometimes defers to formatSci) is
+-- sidestepped entirely by computing the required number of decimal places
+-- directly from the value's exponent (read robustly off a %e string,
+-- since %e always reports the exponent Lua/C computed rather than
+-- re-deriving it ourselves and risking a boundary rounding mismatch) and
+-- formatting with %f at that precision.
+local function formatForcedPlain(value, sig)
+  sig = sig or 3
+  if value == 0 then return { pandoc.Str("0") } end
+  local sciStr = string.format("%." .. (sig - 1) .. "e", value)
+  local exp = tonumber(sciStr:match("e([+-]?%d+)$"))
+  local decimals = sig - 1 - exp
+  if decimals < 0 then decimals = 0 end
+  local s = string.format("%." .. decimals .. "f", value)
+  if s:find("%.") then
+    s = s:gsub("0+$", ""):gsub("%.$", "")
   end
-  return { pandoc.Str(displayMinus(s)) }
+  return { pandoc.Str(finalizePlainString(s)) }
 end
 
 -- decide plain vs. scientific, unless overridden.
 -- mode: nil/"auto" (let %g decide, see formatPlain), "sci", or "plain"
+-- "plain" is a hard override -- it must never silently become scientific,
+-- which is exactly the bug that motivated splitting formatForcedPlain out
+-- from formatPlain rather than reusing it for both cases.
 local function formatNumber(value, mode)
   if value == 0 then return { pandoc.Str("0") } end
   if mode == "sci" then return formatSci(value) end
-  return formatPlain(value) -- handles both "auto" and explicit "plain"
+  if mode == "plain" then return formatForcedPlain(value) end
+  return formatPlain(value) -- "auto"
 end
 
 local function appendAll(dst, list)