local keys=require("keyboard").keys
local shell=require("shell")
local unicode=require("unicode")
local term=require("term")

local args,ops=shell.parse(...)
if#args>1 then
io.write("Usage: ",os.getenv("_"):match("/([^/]+)%.lua$")," <filename>\n")
io.write("- or no args reads stdin\n")
return 1
end

local cat_cmd=table.concat({"cat",...}," ")
if not io.output().tty then
return os.execute(cat_cmd)
end

local preader=io.popen(cat_cmd)
local scrollback=not ops.noback and{}
local bottom=0
local end_of_buffer=false

local width,height=term.getViewport()

local function split(full_line)
local index=1
local parts={}
while true do
local sub=full_line:sub(index,index+width*3)

if#sub<width or unicode.wlen(sub)<=width then
parts[#parts+1]=sub
break
end
parts[#parts+1]=unicode.wtrunc(sub,width+1)
index=index+#parts[#parts]
if index>#full_line then
break
end
end
return parts
end

local function scan(num)
local result={}
local line_count=0
for i=1,num do
local lines={}
if scrollback and(bottom+i)<=#scrollback then
lines={scrollback[bottom+i]}
else
local full_line=preader:read()
if not full_line then preader:close()break end

local buffering=false
for _,line in ipairs(split(full_line))do
if not buffering then
lines[#lines+1]=line
end
if scrollback then
buffering=true
scrollback[#scrollback+1]=line
end
end
end

for _,line in ipairs(lines)do
result[#result+1]=line
line_count=line_count+1
if#result>height then
table.remove(result,1)
end
end

if line_count>=num then
break
end
end
return result,line_count
end

local function status()
if end_of_buffer then
if ops.noback then
os.exit()
end
io.write("(END)")
end
io.write(":")
end

local function goback(n)
if not scrollback then return end
local current_top=bottom-height+1
n=math.min(current_top,n)
if n<1 then return end
local top=current_top-n+1
term.scroll(-n)
term.setCursor(1,1)
for i=1,n do
if i>=height then
break
end
print(scrollback[top+i-1])
end
term.setCursor(1,height)
bottom=bottom-n
end_of_buffer=false
end

local function goforward(n)
term.clearLine()
local update,line_count=scan(n)
for _,line in ipairs(update)do
print(line)
end
if line_count<n then
end_of_buffer=true
end
bottom=bottom+line_count
end

goforward(height-1)

while true do
term.clearLine()
status()
local e,_,_,code=term.pull()
if e=="interrupted"then
break
elseif e=="key_down"then
if code==keys.q then
term.clearLine()
os.exit()
elseif code==keys["end"]then
goforward(math.huge)
elseif code==keys.space or code==keys.pageDown then
goforward(height-1)
elseif code==keys.enter or code==keys.down then
goforward(1)
elseif code==keys.up then
goback(1)
elseif code==keys.pageUp then
goback(height-1)
elseif code==keys.home then
goback(math.huge)
end
end
end