handlers.jl

module Handlers

import ..Request

"""
    Handler

Abstract type for the handler interface that exists for documentation purposes.
A `Handler` is any function of the form `f(req::HTTP.Request) -> HTTP.Response`.
There is no requirement to subtype `Handler` and users should not rely on or dispatch
on `Handler`. A `Handler` function `f` can be passed to [`HTTP.serve`](@ref)
wherein a server will pass each incoming request to `f` to be handled and a response
to be returned. Handler functions are also the inputs to [`Middleware`](@ref) functions
which are functions of the form `f(::Handler) -> Handler`, i.e. they take a `Handler`
function as input, and return a "modified" or enhanced `Handler` function.

For advanced cases, a `Handler` function can also be of the form `f(stream::HTTP.Stream) -> Nothing`.
In this case, the server would be run like `HTTP.serve(f, ...; stream=true)`. For this use-case,
the handler function reads the request and writes the response to the stream directly. Note that
any middleware used with a stream handler also needs to be of the form `f(stream_handler) -> stream_handler`,
i.e. it needs to accept a stream `Handler` function and return a stream `Handler` function.
"""
abstract type Handler end

"""
    Middleware

Abstract type for the middleware interface that exists for documentation purposes.
A `Middleware` is any function of the form `f(::Handler) -> Handler` (ref: [`Handler`](@ref)).
There is no requirement to subtype `Middleware` and users should not rely on or dispatch
on the `Middleware` type. While `HTTP.serve(f, ...)` requires a _handler_ function `f` to be
passed, middleware can be "stacked" to create a chain of functions that are called in sequence,
like `HTTP.serve(base_handler |> cookie_middleware |> auth_middlware, ...)`, where the
`base_handler` `Handler` function is passed to `cookie_middleware`, which takes the handler
and returns a "modified" handler (that parses and stores cookies). This "modified" handler is
then an input to the `auth_middlware`, which further enhances/modifies the handler.
"""
abstract type Middleware end

# tree-based router handler
mutable struct Variable
    name::String
    pattern::Union{Nothing, Regex}
end

const VARREGEX = r"^{([^:{}]+)(?::(.*))?}$"

function Variable(pattern)
    re = Base.match(VARREGEX, pattern)
    if re === nothing
        error("problem parsing path variable for route: `$pattern`")
    end
    pat = re.captures[2]
    return Variable(re.captures[1], pat === nothing ? nothing : Regex(pat))
end

struct Leaf
    method::String
    variables::Vector{Tuple{Int, String}}
    path::String
    handler::Any
end

Base.show(io::IO, x::Leaf) = print(io, "Leaf($(x.method))")

mutable struct Node
    segment::Union{String, Variable}
    exact::Vector{Node} # sorted alphabetically, all x.segment are String
    conditional::Vector{Node} # unsorted; will be applied in source-order; all x.segment are Regex
    wildcard::Union{Node, Nothing} # unconditional variable or wildcard
    doublestar::Union{Node, Nothing} # /** to match any length of path; must be final segment
    methods::Vector{Leaf}
end

Base.show(io::IO, x::Node) = print(io, "Node($(x.segment))")

isvariable(x) = startswith(x, "{") && endswith(x, "}")
segment(x) = isvariable(x) ? Variable(x) : String(x)

Node(x) = Node(x, Node[], Node[], nothing, nothing, Leaf[])
Node() = Node("*")

function find(y, itr; by=identity, eq=(==))
    for (i, x) in enumerate(itr)
        eq(by(x), y) && return i
    end
    return nothing
end

function insert!(node::Node, leaf, segments, i)
    if i > length(segments)
        # time to insert leaf method match node
        j = find(leaf.method, node.methods; by=x->x.method, eq=(x, y) -> x == "*" || x == y)
        if j === nothing
            push!(node.methods, leaf)
        else
            # hmmm, we've seen this route before, warn that we're replacing
            @warn "replacing existing registered route; $(node.methods[j].method) => \"$(node.methods[j].path)\" route with new path = \"$(leaf.path)\""
            node.methods[j] = leaf
        end
        return
    end
    segment = segments[i]
    # @show segment, segment isa Variable
    if segment isa Variable
        # if we're inserting a variable segment, add variable name to leaf vars array
        push!(leaf.variables, (i, segment.name))
    end
    # figure out which kind of node this segment is
    if segment == "*" || (segment isa Variable && segment.pattern === nothing)
        # wildcard node
        if node.wildcard === nothing
            node.wildcard = Node(segment)
        end
        return insert!(node.wildcard, leaf, segments, i + 1)
    elseif segment == "**"
        # double-star node
        if node.doublestar === nothing
            node.doublestar = Node(segment)
        end
        if i < length(segments)
            error("/** double wildcard must be last segment in path")
        end
        return insert!(node.doublestar, leaf, segments, i + 1)
    elseif segment isa Variable
        # conditional node
        # check if we've seen this exact conditional segment before
        j = find(segment.pattern, node.conditional; by=x->x.segment.pattern)
        if j === nothing
            # new pattern
            n = Node(segment)
            push!(node.conditional, n)
        else
            n = node.conditional[j]
        end
        return insert!(n, leaf, segments, i + 1)
    else
        # exact node
        @assert segment isa String
        j = find(segment, node.exact; by=x->x.segment)
        if j === nothing
            # new exact match segment
            n = Node(segment)
            push!(node.exact, n)
            sort!(node.exact; by=x->x.segment)
            return insert!(n, leaf, segments, i + 1)
        else
            # existing exact match segment
            return insert!(node.exact[j], leaf, segments, i + 1)
        end
    end
end

function match(node::Node, method, segments, i)
    # @show node.segment, i, segments
    if i > length(segments)
        if isempty(node.methods)
            return nothing
        end
        j = find(method, node.methods; by=x->x.method, eq=(x, y) -> x == "*" || x == y)
        if j === nothing
            # we return missing here so we can return a 405 instead of 404
            # i.e. we matched the route, but there wasn't a matching method
            return missing
        else
            # return matched leaf node
            return node.methods[j]
        end
    end
    segment = segments[i]
    anymissing = false
    # first check for exact matches
    j = find(segment, node.exact; by=x->x.segment)
    if j !== nothing
        # found an exact match, recurse
        m = match(node.exact[j], method, segments, i + 1)
        anymissing = m === missing
        m = coalesce(m, nothing)
        # @show :exact, m
        if m !== nothing
            return m
        end
    end
    # check for conditional matches
    for node in node.conditional
        # @show node.segment.pattern, segment
        if Base.match(node.segment.pattern, segment) !== nothing
            # matched a conditional node, recurse
            m = match(node, method, segments, i + 1)
            anymissing = m === missing
            m = coalesce(m, nothing)
            if m !== nothing
                return m
            end
        end
    end
    if node.wildcard !== nothing
        m = match(node.wildcard, method, segments, i + 1)
        anymissing = m === missing
        m = coalesce(m, nothing)
        if m !== nothing
            return m
        end
    end
    if node.doublestar !== nothing
        m = match(node.doublestar, method, segments, length(segments) + 1)
        anymissing = m === missing
        m = coalesce(m, nothing)
        if m !== nothing
            return m
        end
    end
    return anymissing ? missing : nothing
end

"""
    HTTP.Router(_404, _405, middleware=nothing)

Define a router object that maps incoming requests by path to registered routes and
associated handlers. Paths can be registered using [`HTTP.register!`](@ref). The router
object itself is a "request handler" that can be called like:
```
r = HTTP.Router()
resp = r(request)
```

Which will inspect the `request`, find the matching, registered handler from the url,
and pass the request on to be handled further.

See [`HTTP.register!`](@ref) for additional information on registering handlers based on routes.

If a request doesn't have a matching, registered handler, the `_404` handler is called which,
by default, returns a `HTTP.Response(404)`. If a route matches the path, but not the method/verb
(e.g. there's a registerd route for "GET /api", but the request is "POST /api"), then the `_405`
handler is called, which by default returns `HTTP.Response(405)` (method not allowed).

A `middleware` ([`Middleware`](@ref)) can optionally be provided as well, which will be called
after the router has matched the request to a route, but before the route's handler is called.
This provides a "hook" for matched routes that can be helpful for metric tracking, logging, etc.
Note that the middleware is only called if the route is matched; for the 404 and 405 cases,
users should wrap those handlers in the `middleware` manually.
"""
struct Router{T, S, F}
    _404::T
    _405::S
    routes::Node
    middleware::F
end

default404(::Request) = Response(404)
default405(::Request) = Response(405)
# default404(s::Stream) = setstatus(s, 404)
# default405(s::Stream) = setstatus(s, 405)

Router(_404=default404, _405=default405, middleware=nothing) = Router(_404, _405, Node(), middleware)

"""
    HTTP.register!(r::Router, method, path, handler)
    HTTP.register!(r::Router, path, handler)

Register a handler function that should be called when an incoming request matches `path`
and the optionally provided `method` (if not provided, any method is allowed). Can be used
to dynamically register routes. When a registered route is matched, the original route string
is stored in the `request.context[:route]` variable.
The following path types are allowed for matching:
  * `/api/widgets`: exact match of static strings
  * `/api/*/owner`: single `*` to wildcard match anything for a single segment
  * `/api/widget/{id}`: Define a path variable `id` that matches any valued provided for this segment; path variables are available in the request context like `HTTP.getparams(req)["id"]`
  * `/api/widget/{id:[0-9]+}`: Define a path variable `id` that does a regex match for integers for this segment
  * `/api/**`: double wildcard matches any number of trailing segments in the request path; the double wildcard must be the last segment in the path
"""
function register! end

function register!(r::Router, method, path, handler)
    segments = map(segment, split(path, '/'; keepempty=false))
    if r.middleware !== nothing
        handler = r.middleware(handler)
    end
    insert!(r.routes, Leaf(method, Tuple{Int, String}[], path, handler), segments, 1)
    return
end

register!(r::Router, path, handler) = register!(r, "*", path, handler)

const Params = Dict{String, String}

function gethandler(r::Router, req::Request)
    url = req.uri
    segments = split(url.path, '/'; keepempty=false)
    leaf = match(r.routes, req.method, segments, 1)
    params = Params()
    if leaf isa Leaf
        # @show leaf.variables, segments
        if !isempty(leaf.variables)
            # we have variables to fill in
            for (i, v) in leaf.variables
                params[v] = segments[i]
            end
        end
        return leaf.handler, leaf.path, params
    end
    return leaf, "", params
end

# function (r::Router)(stream::Stream{<:Request})
#     req = stream.message
#     handler, route, params = gethandler(r, req)
#     if handler === nothing
#         # didn't match a registered route
#         return r._404(stream)
#     elseif handler === missing
#         # matched the path, but method not supported
#         return r._405(stream)
#     else
#         req.context[:route] = route
#         if !isempty(params)
#             req.context[:params] = params
#         end
#         return handler(stream)
#     end
# end

function (r::Router)(req::Request)
    handler, route, params = gethandler(r, req)
    if handler === nothing
        # didn't match a registered route
        return r._404(req)
    elseif handler === missing
        # matched the path, but method not supported
        return r._405(req)
    else
        req.context[:route] = route
        if !isempty(params)
            req.context[:params] = params
        end
        return handler(req)
    end
end

"""
    HTTP.getroute(req) -> String

Retrieve the original route registration string for a request after its url has been
matched against a router. Helpful for metric logging to ignore matched variables in
a path and only see the registered routes.
"""
getroute(req) = Base.get(req.context, :route, nothing)

"""
    HTTP.getparams(req) -> Dict{String, String}

Retrieve any matched path parameters from the request context.
If a path was registered with a router via `HTTP.register!` like
"/api/widget/{id}", then the path parameters are available in the request context
and can be retrieved like `id = HTTP.getparams(req)["id"]`.
"""
getparams(req) = Base.get(req.context, :params, nothing)

"""
    HTTP.getparam(req, name, default=nothing) -> String

Retrieve a matched path parameter with name `name` from request context.
If a path was registered with a router via `HTTP.register!` like
"/api/widget/{id}", then the path parameter can be retrieved like `id = HTTP.getparam(req, "id").
"""
function getparam(req, name, default=nothing)
    params = getparams(req)
    params === nothing && return default
    return Base.get(params, name, default)
end

"""
    HTTP.Handlers.cookie_middleware(handler) -> handler

Middleware that parses and stores any cookies in the incoming
request in the request context. Cookies can then be retrieved by calling
[`HTTP.getcookies(req)`](@ref) in subsequent middlewares/handlers.
"""
function cookie_middleware(handler)
    function (req)
        if !haskey(req.context, :cookies)
            req.context[:cookies] = Cookies.cookies(req)
        end
        return handler(req)
    end
end

"""
    HTTP.getcookies(req) -> Vector{Cookie}

Retrieve any parsed cookies from a request context. Cookies
are expected to be stored in the `req.context[:cookies]` of the
request context as implemented in the [`HTTP.Handlers.cookie_middleware`](@ref)
middleware.
"""
getcookies(req) = Base.get(() -> Cookie[], req.context, :cookies)

end # module Handlers