Fix numerous off-by-one errors in help program

We clamped various values to the height of the screen, rather than the
height of the content box (height-1). We didn't notice this most of the
time as the last line of a file is empty - it only really mattered when
a file was the same height as the computer's screen.

We now do the following:
 - Strip the trailing new line from a file when reading.
 - Replace most usages of height with height-1.

src/main/resources/data/computercraft/lua/rom/programs/help.lua

@@ -146,14 +146,17 @@ end
 
 local contents = file:read("*a")
 file:close()
+-- Trim trailing newlines from the file to avoid displaying a blank line.
+if contents:sub(-1) == "\n" then contents:sub(1, -2) end
 
 local word_wrap = sFile:sub(-3) == ".md" and word_wrap_markdown or word_wrap_basic
 local width, height = term.getSize()
+local content_height = height - 1 -- Height of the content box.
 local lines, fg, bg, sections = word_wrap(contents, width)
 local print_height = #lines
 
 -- If we fit within the screen, just display without pagination.
-if print_height <= height then
+if print_height <= content_height then
     local _, y = term.getCursorPos()
     for i = 1, print_height do
         if y + i - 1 > height then
@@ -201,7 +204,7 @@ end
 
 
 local function draw()
-    for y = 1, height - 1 do
+    for y = 1, content_height do
         term.setCursorPos(1, y)
         if y + offset > print_height then
             -- Should only happen if we resize the terminal to a larger one
@@ -228,14 +231,14 @@ while true do
         if param == keys.up and offset > 0 then
             offset = offset - 1
             draw()
-        elseif param == keys.down and offset < print_height - height then
+        elseif param == keys.down and offset < print_height - content_height then
             offset = offset + 1
             draw()
         elseif param == keys.pageUp and offset > 0 then
-            offset = math.max(offset - height + 2, 0)
+            offset = math.max(offset - content_height + 1, 0)
             draw()
-        elseif param == keys.pageDown and offset < print_height - height then
-            offset = math.min(offset + height - 2, print_height - height)
+        elseif param == keys.pageDown and offset < print_height - content_height then
+            offset = math.min(offset + content_height - 1, print_height - content_height)
             draw()
         elseif param == keys.home then
             offset = 0
@@ -247,7 +250,7 @@ while true do
             offset = sections[current_section + 1].offset
             draw()
         elseif param == keys["end"] then
-            offset = print_height - height
+            offset = print_height - content_height
             draw()
         elseif param == keys.q then
             sleep(0) -- Super janky, but consumes stray "char" events.
@@ -257,7 +260,7 @@ while true do
         if param < 0 and offset > 0 then
             offset = offset - 1
             draw()
-        elseif param > 0 and offset < print_height - height then
+        elseif param > 0 and offset <= print_height - content_height then
             offset = offset + 1
             draw()
         end
@@ -270,7 +273,8 @@ while true do
         end
 
         width, height = new_width, new_height
-        offset = math.max(math.min(offset, print_height - height), 0)
+        content_height = height - 1
+        offset = math.max(math.min(offset, print_height - content_height), 0)
         draw()
         draw_menu()
     elseif event == "terminate" then

src/test/resources/test-rom/spec/programs/help_spec.lua

@@ -1,8 +1,46 @@
 local capture = require "test_helpers".capture_program
+local with_window_lines = require "test_helpers".with_window_lines
 
 describe("The help program", function()
+    local function stub_help(content)
+        local name = "/help_file.txt"
+        io.open(name, "wb"):write(content):close()
+        stub(help, "lookup", function() return name end)
+    end
+
+    local function capture_help(width, height, content)
+        stub_help(content)
+
+        local co = coroutine.create(shell.run)
+        local window = with_window_lines(width, height, function()
+            local ok, err = coroutine.resume(co, "help topic")
+            if not ok then error(err, 0) end
+        end)
+        return coroutine.status(co) == "dead", window
+    end
+
     it("errors when there is no such help file", function()
         expect(capture(stub, "help nothing"))
             :matches { ok = true, error = "No help available\n", output = "" }
     end)
+
+    it("prints a short file directly", function()
+        local dead, output = capture_help(10, 3, "a short\nfile")
+        expect(dead):eq(true)
+        expect(output):same {
+            "a short   ",
+            "file      ",
+            "          ",
+        }
+    end)
+
+    it("launches the viewer for a longer file", function()
+        local dead, output = capture_help(10, 3, "a longer\nfile\nwith content")
+        expect(dead):eq(false)
+        expect(output):same {
+            "a longer  ",
+            "file      ",
+            "Help: topi",
+        }
+    end)
 end)

src/test/resources/test-rom/spec/test_helpers.lua

@@ -56,7 +56,22 @@ local function with_window(width, height, fn)
     return redirect
 end
 
+--- Run a function redirecting to a new window with the given dimensions,
+-- returning the content of the window.
+--
+-- @tparam number width The window's width
+-- @tparam number height The window's height
+-- @tparam function() fn The action to run
+-- @treturn {string...} The content of the window.
+local function with_window_lines(width, height, fn)
+    local window = with_window(width, height, fn)
+    local out = {}
+    for i = 1, height do out[i] = window.getLine(i) end
+    return out
+end
+
 return {
     capture_program = capture_program,
     with_window = with_window,
+    with_window_lines = with_window_lines,
 }