#' Calculate initial predicted environmental concentrations in surface water due to drainage using the UK method
#'
#' This implements the method specified in the UK data requirements handbook and was checked against the spreadsheet
#' published on the CRC website. Degradation between the end (30 April) and the start (1 October) of 
#' the drainage period is taken into account if
#' `latest_application` is specified and the degradation parameters are given either as a `soil_DT50` or a `model`.
#'
#' @param rate Application rate in g/ha or with a compatible unit specified
#' with the units package
#' @param interception The fraction of the application rate that does not reach the soil
#' @param Koc The sorption coefficient normalised to organic carbon in L/kg or a unit specified
#' with the units package
#' @param latest_application Latest application date, formatted as e.g. "01 July"
#' @param soil_DT50 Soil degradation half-life, if SFO kinetics are to be used, in
#' days or a time unit specified with the units package
#' @param model The soil degradation model to be used. Either one of "FOMC",
#'   "DFOP", "HS", or "IORE", or an mkinmod object
#' @param model_parms A named numeric vector containing the model parameters
#' @return The predicted concentration in surface water in µg/L
#' @references HSE's Chemicals Regulation Division (CRD) Active substance
#'   PECsw calculations (for UK specific authorisation requests)
#'   \url{https://www.hse.gov.uk/pesticides/data-requirements-handbook/fate/pecsw-sed-via-drainflow.htm}
#'   accessed 2026-02-13
#'
#'   PECsw/sed spray drift and tier 1 drainflow calculator Version 2.1.1 (2025) Spreadsheet published at
#'   \url{https://www.hse.gov.uk/pesticides/assets/docs/PEC%20sw-sed%20(spraydrift).xlsx)}
#'   accessed 2026-02-13
#' @export
#' @author Johannes Ranke
#' @examples
#' PEC_sw_drainage_UK(150, Koc = 100)
#' PEC_sw_drainage_UK(60, interception = 0.5, Koc = 550,
#'   latest_application = "01 July", soil_DT50 = 200)
PEC_sw_drainage_UK <- function(rate,
  interception = 0, Koc,
  latest_application = NULL, soil_DT50 = NULL,
  model = NULL, model_parms = NULL)
{
  # Set default units if not specified and convert to units used in the calculations
  if (!inherits(rate, "units")) rate <- set_units(rate, "g/ha")
  rate_g_ha <- as.numeric(set_units(rate, "g/ha"))

  if (!inherits(Koc, "units")) Koc <- set_units(Koc, "L/kg")
  Koc_L_kg <- as.numeric(set_units(Koc, "L/kg"))

  if (!missing(soil_DT50)) {
    if (!inherits(soil_DT50, "units")) {
      soil_DT50_d <- soil_DT50
    }
    soil_DT50_d <- as.numeric(set_units(soil_DT50, "d"))
  }

  percentage_lost <- SSLRC_mobility_classification(Koc_L_kg)[[2]]
  amount_available <- rate_g_ha * (1 - interception) # amount in g for 1 ha

  if (!missing(latest_application)) {
    lct <- Sys.getlocale("LC_TIME")
    tmp <- Sys.setlocale("LC_TIME", "C")
    if (latest_application == "29 February") { # Use a leap year
      ref_year <- 2000
    } else { ref_year <- 1999} # Use a non-leap year
    latest <- as.Date(paste(latest_application, ref_year), "%d %b %Y")
    if (is.na(latest)) stop("Please specify the latest application in the format '%d %b', e.g. '01 July'")
    tmp <- Sys.setlocale("LC_TIME", lct)
    
    drainage_date <- drainage_date_UK(latest)
    degradation_time <- as.numeric(difftime(drainage_date, latest, units = "days"))
    
    if (degradation_time > 0) {
      if (!missing(soil_DT50)) {
        k = log(2)/soil_DT50_d
        amount_available <- amount_available * exp(-k * degradation_time)
        if (!missing(model)) stop("You already supplied a soil_DT50 value, implying SFO kinetics")
      }
      if (!missing(model)) {
        fraction_left <- pfm_degradation(model, parms = model_parms,
                                         times = degradation_time)[1, "parent"]
        amount_available <- fraction_left * amount_available
      }
    }
  }

  volume = 130000 # L/ha
  PEC = set_units(1e6 * (percentage_lost/100) * amount_available / volume, "\u00B5g/L")
  return(PEC)
}

#' @rdname PEC_sw_drainage_UK
#' @param application_date Application date
#' @export
#' @examples
#' drainage_date_UK("2023-07-10")
#' drainage_date_UK("2020-12-01")
#' drainage_date_UK(as.Date("2022-01-15"))
drainage_date_UK <- function(application_date) {
  year <- substr(application_date, 1, 4)
  drainage_end <- as.Date(paste0(year, "-04-30"))
  drainage_start <- as.Date(paste0(year, "-10-01"))
  if (application_date <= drainage_end | application_date >= drainage_start) {
    drainage_date <- application_date
  } else {
    drainage_date <- drainage_start
  }
  return(drainage_date)
}