content: {-|
  
  Convert amounts to some related value in various ways. This involves
  looking up historical market prices (exchange rates) between commodities.
  
  -}
  
  {-# LANGUAGE LambdaCase #-}
  {-# LANGUAGE NamedFieldPuns #-}
  {-# LANGUAGE OverloadedStrings #-}
  {-# LANGUAGE RecordWildCards #-}
  {-# LANGUAGE ScopedTypeVariables #-}
  {-# LANGUAGE DeriveGeneric #-}
  
  module Hledger.Data.Valuation (
     ConversionOp(..)
    ,ValuationType(..)
    ,PriceOracle
    ,journalPriceOracle
    ,mixedAmountToCost
    ,mixedAmountApplyValuation
    ,mixedAmountValueAtDate
    ,mixedAmountApplyGain
    ,mixedAmountGainAtDate
    ,marketPriceReverse
    ,priceDirectiveToMarketPrice
    ,amountPriceDirectiveFromCost
    ,valuationTypeValuationCommodity
    -- ,priceLookup
    ,tests_Valuation
  )
  where
  
  import Control.Applicative ((<|>))
  import Data.Function ((&), on)
  import Data.List (partition, intercalate, sortBy)
  import Data.List.Extra (nubSortBy)
  import Data.Map qualified as M
  import Data.Set qualified as S
  import Data.Text qualified as T
  import Data.Time.Calendar (Day, fromGregorian)
  import Data.MemoUgly (memo)
  import GHC.Generics (Generic)
  import Safe (headMay, lastMay)
  
  import Hledger.Utils
  import Hledger.Data.Types
  import Hledger.Data.Amount
  import Hledger.Data.Dates (nulldate)
  import Text.Printf (printf)
  import Data.Decimal (decimalPlaces, roundTo, Decimal)
  import Data.Word (Word8)
  
  
  ------------------------------------------------------------------------------
  -- Types
  
  -- | Which operation to perform on conversion transactions.
  -- (There was also an "infer equity postings" operation, but that is now done 
  -- earlier, in journal finalisation.)
  data ConversionOp = NoConversionOp | ToCost
    deriving (Show,Eq)
  
  -- | What kind of value conversion should be done on amounts ?
  -- CLI: --value=then|end|now|DATE[,COMM]
  data ValuationType =
      AtThen     (Maybe CommoditySymbol)  -- ^ convert to default or given valuation commodity, using market prices at each posting's date
    | AtEnd      (Maybe CommoditySymbol)  -- ^ convert to default or given valuation commodity, using market prices at period end(s)
    | AtNow      (Maybe CommoditySymbol)  -- ^ convert to default or given valuation commodity, using current market prices
    | AtDate Day (Maybe CommoditySymbol)  -- ^ convert to default or given valuation commodity, using market prices on some date
    deriving (Show,Eq)
  
  valuationTypeValuationCommodity :: ValuationType -> Maybe CommoditySymbol
  valuationTypeValuationCommodity = \case
      AtThen   (Just c) -> Just c
      AtEnd    (Just c) -> Just c
      AtNow    (Just c) -> Just c
      AtDate _ (Just c) -> Just c
      _                 -> Nothing
  
  -- | A price oracle is a magic memoising function that efficiently
  -- looks up market prices (exchange rates) from one commodity to
  -- another (or if unspecified, to a default valuation commodity) on a
  -- given date.
  type PriceOracle = (Day, CommoditySymbol, Maybe CommoditySymbol) -> Maybe (CommoditySymbol, Quantity)
  
  -- | Generate a price oracle (memoising price lookup function) from a
  -- journal's directive-declared and transaction-inferred market
  -- prices. For best performance, generate this only once per journal,
  -- reusing it across reports if there are more than one, as
  -- compoundBalanceCommand does.
  -- The boolean argument is whether to infer market prices from
  -- transactions or not.
  journalPriceOracle :: Bool -> Journal -> PriceOracle
  journalPriceOracle infer Journal{jpricedirectives, jinferredmarketprices} =
    let
      declaredprices = map priceDirectiveToMarketPrice jpricedirectives
      inferredprices =
        (if infer then jinferredmarketprices else [])
        & dbg2Msg ("use prices inferred from costs? " <> if infer then "yes" else "no")
      makepricegraph = memo $ makePriceGraph declaredprices inferredprices
    in
      memo $ uncurry3 $ priceLookup makepricegraph
  
  priceDirectiveToMarketPrice :: PriceDirective -> MarketPrice
  priceDirectiveToMarketPrice PriceDirective{..} =
    MarketPrice{ mpdate = pddate
               , mpfrom = pdcommodity
               , mpto   = acommodity pdamount
               , mprate = aquantity pdamount
               }
  
  -- | Infer a market price from the given amount and its cost (if any),
  -- and make a corresponding price directive on the given date.
  -- The price's display precision will be set to show all significant
  -- decimal digits; or if they seem to be infinite, defaultPrecisionLimit.
  amountPriceDirectiveFromCost :: Day -> Amount -> Maybe PriceDirective
  amountPriceDirectiveFromCost d amt@Amount{acommodity=fromcomm, aquantity=n} = case acost amt of
      Just (UnitCost u)           -> Just $ pd{pdamount=u}
      Just (TotalCost t) | n /= 0 -> Just $ pd{pdamount=u}
        where u = amountSetFullPrecisionUpTo Nothing $ divideAmount n t
      _                            -> Nothing
    where
      pd = PriceDirective{pdsourcepos=nullsourcepos, pddate=d, pdcommodity=fromcomm, pdamount=nullamt}
  
  ------------------------------------------------------------------------------
  -- Converting things to value
  
  -- | Convert all component amounts to cost/selling price if requested, and style them.
  mixedAmountToCost :: M.Map CommoditySymbol AmountStyle -> ConversionOp -> MixedAmount -> MixedAmount
  mixedAmountToCost styles cost = mapMixedAmount (amountToCost styles cost)
  
  -- | Apply a specified valuation to this mixed amount, using the
  -- provided price oracle, commodity styles, and reference dates.
  -- See amountApplyValuation.
  mixedAmountApplyValuation :: PriceOracle -> M.Map CommoditySymbol AmountStyle -> Day -> Day -> Day -> ValuationType -> MixedAmount -> MixedAmount
  mixedAmountApplyValuation priceoracle styles periodlast today postingdate v =
    mapMixedAmount (amountApplyValuation priceoracle styles periodlast today postingdate v)
  
  -- | Convert an Amount to its cost if requested, and style it appropriately.
  amountToCost :: M.Map CommoditySymbol AmountStyle -> ConversionOp -> Amount -> Amount
  amountToCost styles ToCost         = styleAmounts styles . amountCost
  amountToCost _      NoConversionOp = id
  
  -- | Apply a specified valuation to this amount, using the provided
  -- price oracle, and reference dates. Also fix up its display style
  -- using the provided commodity styles.
  --
  -- When the valuation requires converting to another commodity, a
  -- valuation (conversion) date is chosen based on the valuation type
  -- and the provided reference dates. It will be one of:
  --
  -- - the date of the posting itself (--value=then)
  --
  -- - the provided "period end" date - this is typically the last day
  --   of a subperiod (--value=end with a multi-period report), or of
  --   the specified report period or the journal (--value=end with a
  --   single-period report).
  --
  -- - the provided "today" date (--value=now).
  --
  -- - a fixed date specified by the ValuationType itself
  --   (--value=DATE).
  --
  -- This is all a bit complicated. See the reference doc at
  -- https://hledger.org/hledger.html#effect-of-valuation-on-reports
  -- (hledger_options.m4.md "Effect of valuation on reports"), and #1083.
  --
  amountApplyValuation :: PriceOracle -> M.Map CommoditySymbol AmountStyle -> Day -> Day -> Day -> ValuationType -> Amount -> Amount
  amountApplyValuation priceoracle styles periodlast today postingdate v a =
    case v of
      AtThen    mc      -> amountValueAtDate priceoracle styles mc postingdate a
      AtEnd     mc      -> amountValueAtDate priceoracle styles mc periodlast a
      AtNow     mc      -> amountValueAtDate priceoracle styles mc today a
      AtDate d  mc      -> amountValueAtDate priceoracle styles mc d a
  
  -- | Find the market value of each component amount in the given
  -- commodity, or its default valuation commodity, at the given
  -- valuation date, using the given market price oracle.
  -- When market prices available on that date are not sufficient to
  -- calculate the value, amounts are left unchanged.
  mixedAmountValueAtDate :: PriceOracle -> M.Map CommoditySymbol AmountStyle -> Maybe CommoditySymbol -> Day -> MixedAmount -> MixedAmount
  mixedAmountValueAtDate priceoracle styles mc d = mapMixedAmount (amountValueAtDate priceoracle styles mc d)
  
  -- | Find the market value of this amount in the given valuation
  -- commodity if any, otherwise the default valuation commodity, at the
  -- given valuation date. (The default valuation commodity is the
  -- commodity of the latest applicable market price before the
  -- valuation date.)
  --
  -- The returned amount will have its commodity's canonical style applied,
  -- (with soft display rounding).
  --
  -- If the market prices available on that date are not sufficient to
  -- calculate this value, the amount is left unchanged.
  --
  amountValueAtDate :: PriceOracle -> M.Map CommoditySymbol AmountStyle -> Maybe CommoditySymbol -> Day -> Amount -> Amount
  amountValueAtDate priceoracle styles mto d a =
    let lbl = lbl_ "amountValueAtDate" in
    case priceoracle (d, acommodity a, mto) of
      Nothing           -> a
      Just (comm, rate) ->
        nullamt{acommodity=comm, aquantity=rate * aquantity a}
  
        -- Manage style and precision of the new amount. Initially:
        --  rate is a Decimal with the internal precision of the original market price declaration.
        --  aquantity is a Decimal with a's internal precision.
        --  The calculated value's internal precision may be different from these.
        --  Its display precision will be that of nullamt (0).
        -- Now apply the standard display style for comm (if there is one)
        & styleAmounts styles
        -- set the display precision to match the internal precision (showing all digits),
        -- unnormalised (don't strip trailing zeros);
        -- but if it looks like an infinite decimal, limit the precision to 8.
        & amountSetFullPrecisionUpTo Nothing
        & dbg9With (lbl "calculated value".showAmount)
  
  -- | Calculate the gain of each component amount, that is the difference
  -- between the valued amount and the value of the cost basis (see
  -- mixedAmountApplyValuation).
  --
  -- If the commodity we are valuing in is not the same as the commodity of the
  -- cost, this will value the cost at the same date as the primary amount. This
  -- may not be what you want; for example you may want the cost valued at the
  -- posting date. If so, let us know and we can change this behaviour.
  mixedAmountApplyGain :: PriceOracle -> M.Map CommoditySymbol AmountStyle -> Day -> Day -> Day -> ValuationType -> MixedAmount -> MixedAmount
  mixedAmountApplyGain priceoracle styles periodlast today postingdate v ma =
    mixedAmountApplyValuation priceoracle styles periodlast today postingdate v $ ma `maMinus` mixedAmountCost ma
  
  -- | Calculate the gain of each component amount, that is the
  -- difference between the valued amount and the value of the cost basis.
  --
  -- If the commodity we are valuing in is not the same as the commodity of the
  -- cost, this will value the cost at the same date as the primary amount. This
  -- may not be what you want; for example you may want the cost valued at the
  -- posting date. If so, let us know and we can change this behaviour.
  mixedAmountGainAtDate :: PriceOracle -> M.Map CommoditySymbol AmountStyle -> Maybe CommoditySymbol -> Day -> MixedAmount -> MixedAmount
  mixedAmountGainAtDate priceoracle styles mto d ma =
    mixedAmountValueAtDate priceoracle styles mto d $ ma `maMinus` mixedAmountCost ma
  
  ------------------------------------------------------------------------------
  -- Market price lookup
  
  -- | Given a memoising price graph generator, a valuation date, a
  -- source commodity and an optional valuation commodity, find the
  -- value on that date of one unit of the source commodity in the
  -- valuation commodity, or in a default valuation commodity. Returns
  -- the valuation commodity that was specified or chosen, and the
  -- quantity of it that one unit of the source commodity is worth. Or
  -- if no applicable market price can be found or calculated, or if the
  -- source commodity and the valuation commodity are the same, returns
  -- Nothing.
  --
  -- See makePriceGraph for how prices are determined.
  -- Note that both market prices and default valuation commodities can
  -- vary with valuation date, since that determines which market prices
  -- are visible.
  --
  priceLookup :: (Day -> PriceGraph) -> Day -> CommoditySymbol -> Maybe CommoditySymbol -> Maybe (CommoditySymbol, Quantity)
  priceLookup makepricegraph d from mto =
    -- trace ("priceLookup ("++show d++", "++show from++", "++show mto++")") $
    let
      PriceGraph{pgEdges=forwardprices
                ,pgEdgesRev=allprices
                ,pgDefaultValuationCommodities=defaultdests
                } =
        dbg1Msg ("valuation date: "++show d) $ makepricegraph d
      mto' = mto <|> mdefaultto
        where
          mdefaultto = dbg1 ("default valuation commodity for "++T.unpack from) $
                       M.lookup from defaultdests
    in
      case mto' of
        Nothing            -> Nothing
        Just to | to==from -> Nothing
        Just to            ->
          -- We have a commodity to convert to. Find the most direct price available,
          -- according to the rules described in makePriceGraph.
          let
            msg = printf "seeking %s to %s price" (showCommoditySymbol from) (showCommoditySymbol to)
            prices =
              (dbg2Msg (msg++" using forward prices") $
               dbg2Msg ("forward prices:\n" <> showMarketPrices forwardprices) $
               pricesShortestPath from to forwardprices)
              <|>
              (dbg2Msg (msg++" using forward and reverse prices") $
               dbg2Msg ("forward and reverse prices:\n" <> showMarketPrices allprices) $
               pricesShortestPath from to $ dbg5 "all forward and reverse prices" allprices)
          in case prices of
            Nothing -> Nothing
            Just [] -> Nothing
            Just ps -> Just (mpto $ last ps, rate)
              where
                rates = map mprate ps
                rate =
                  -- aggregate all the prices into one
                  product rates
                  -- product (Decimal's Num instance) normalises, stripping trailing zeros.
                  -- But we want to preserve even those, since the number of decimal digits
                  -- here will guide amountValueAtDate in setting the Amount display precision later.
                  -- So we restore them. Or rather, we ensure as many decimal digits as the maximum seen among rates.
                  -- (Some prices might end up more precise than they were, but that seems harmless here.)
                  & setMinDecimalPlaces (maximum $ map decimalPlaces rates)
  
  -- Ensure this Decimal has at least this many decimal places, adding trailing zeros if necessary.
  setMinDecimalPlaces :: Word8 -> Decimal -> Decimal
  setMinDecimalPlaces n d
    | decimalPlaces d < n = roundTo n d  -- too few, add some zeros
    | otherwise           = d            -- more than enough, keep as-is
  
  tests_priceLookup =
    let
      p y m d from q to = MarketPrice{mpdate=fromGregorian y m d, mpfrom=from, mpto=to, mprate=q}
      ps1 = [
         p 2000 01 01 "A" 10 "B"
        ,p 2000 01 01 "B" 10 "C"
        ,p 2000 01 01 "C" 10 "D"
        ,p 2000 01 01 "E"  2 "D"
        ,p 2001 01 01 "A" 11 "B"
        ]
      makepricegraph = makePriceGraph ps1 []
    in testCase "priceLookup" $ do
      priceLookup makepricegraph (fromGregorian 1999 01 01) "A" Nothing    @?= Nothing
      priceLookup makepricegraph (fromGregorian 2000 01 01) "A" Nothing    @?= Just ("B",10)
      priceLookup makepricegraph (fromGregorian 2000 01 01) "B" (Just "A") @?= Just ("A",0.1)
      priceLookup makepricegraph (fromGregorian 2000 01 01) "A" (Just "E") @?= Just ("E",500)
  
  ------------------------------------------------------------------------------
  -- Market price graph
  -- built directly with MarketPrices for now, probably space-inefficient
  
  type Edge = MarketPrice
  type Path = [Edge]
  
  data PriceGraph = PriceGraph {
     pgDate :: Day
      -- ^ The date on which these prices are in effect.
    ,pgEdges :: [Edge]
      -- ^ "Forward" exchange rates between commodity pairs, either
      --   declared by P directives or (with --infer-market-prices) inferred from costs,
      --   forming the edges of a directed graph.  
    ,pgEdgesRev :: [Edge]
      -- ^ The same edges, plus any additional edges that can be
      --   inferred by reversing them and inverting the rates.
      --
      --   In both of these there will be at most one edge between each
      --   directed pair of commodities, eg there can be one USD->EUR and one EUR->USD.
      --
    ,pgDefaultValuationCommodities :: M.Map CommoditySymbol CommoditySymbol
      -- ^ The default valuation commodity for each source commodity.
      --   These are used when a valuation commodity is not specified
      --   (-V). They are the destination commodity of each source commodity's
      --   latest (declared or inferred, but not reverse) market price
      --   (on the date of this graph).
    }
    deriving (Show,Generic)
  
  -- | Find the shortest path and corresponding conversion rate, if any, 
  -- from one commodity to another using the provided market prices which
  -- form the edges of a directed graph. There should be at most one edge
  -- between each directed pair of commodities, eg there can be one
  -- USD->EUR price and one EUR->USD price.
  pricesShortestPath :: CommoditySymbol -> CommoditySymbol -> [Edge] -> Maybe Path
  pricesShortestPath start end edges =
    -- at --debug=2 +, print the pretty path and also the detailed prices
    let label = printf "shortest path from %s to %s: " (showCommoditySymbol start) (showCommoditySymbol end) in
    fmap (dbg2With (("price chain:\n"++).showMarketPrices)) $
    dbg2With ((label++).(maybe "none" (pshowpath ""))) $
    find [([],edges)]
  
    where
      -- Find the first and shortest complete path using a breadth-first search.
      find :: [(Path,[Edge])] -> Maybe Path
      find paths =
        case concatMap extend paths of
          [] -> Nothing 
          _ | pathlength > maxpathlength -> 
            -- Print a non-fatal warning to stderr, something we usually avoid.
            warn ("gave up searching for a price chain at length "++show maxpathlength++", please report a bug")
            Nothing
            where 
              pathlength = 2 + maybe 0 (length . fst) (headMay paths)
              maxpathlength = 1000
          paths' -> 
            case completepaths of
                  p:_ -> Just p  -- the left-most complete path at this length
                  []  -> find paths'
            where completepaths = [p | (p,_) <- paths', (mpto <$> lastMay p) == Just end]
  
      -- Use all applicable edges from those provided to extend this path by one step,
      -- returning zero or more new (path, remaining edges) pairs.
      extend :: (Path,[Edge]) -> [(Path,[Edge])]
      extend (path,unusededges) =
        let
          pathnodes = start : map mpto path
          pathend = maybe start mpto $ lastMay path
          (nextedges,remainingedges) = partition ((==pathend).mpfrom) unusededges
        in
          [ (path', remainingedges')
          | e <- nextedges
          , let path' = dbgpath "trying" $ path ++ [e]  -- PERF prepend ?
          , let pathnodes' = mpto e : pathnodes
          , let remainingedges' = [r | r <- remainingedges, mpto r `notElem` pathnodes' ]
          ]
  
  -- debug helpers
  dbgpath  label = dbg2With (pshowpath label)
  -- dbgedges label = dbg2With (pshowedges label)
  pshowpath label = \case
    []      -> prefix label ""
    p@(e:_) -> prefix label $ pshownode (mpfrom e) ++ ">" ++ intercalate ">" (map (pshownode . mpto) p)
  -- pshowedges label = prefix label . intercalate ", " . map (pshowedge "")
  -- pshowedge label MarketPrice{..} = pshowedge' label mpfrom mpto
  -- pshowedge' label from to = prefix label $ pshownode from ++ ">" ++ pshownode to
  pshownode = T.unpack . showCommoditySymbol
  prefix l = if null l then (""++) else ((l++": ")++)
  
  -- | A snapshot of the known exchange rates between commodity pairs at a given date.
  -- This is a home-made version, more tailored to our needs.
  -- | Build the graph of commodity conversion prices for a given day.
  -- Converts a list of declared market prices in parse order, and a
  -- list of transaction-inferred market prices in parse order, to:
  --
  -- 1. a graph of all known exchange rates declared or inferred from 
  -- one commodity to another in effect on that day
  --
  -- 2. a second graph which includes any additional exchange rates
  -- that can be inferred by reversing known rates
  --
  -- 3. a map of each commodity's default valuation commodity, if any.
  --
  -- These allow price lookup and valuation to be performed as
  -- described in hledger.m4.md -> Valuation:
  --
  -- "hledger looks for a market price (exchange rate) from commodity A
  -- to commodity B in one or more of these ways, in this order of
  -- preference:
  --
  -- 1. A *declared market price* or *inferred market price*:
  --    A's latest market price in B on or before the valuation date
  --    as declared by a P directive, or (with the `--infer-market-prices` flag)
  --    inferred from transaction prices.
  --   
  -- 2. A *reverse market price*:
  --    the inverse of a declared or inferred market price from B to A.
  -- 
  -- 3. A *a forward chain of market prices*:
  --    a synthetic price formed by combining the shortest chain of
  --    "forward" (only 1 above) market prices, leading from A to B.
  --
  -- 4. A *any chain of market prices*:
  --    a chain of any market prices, including both forward and
  --    reverse prices (1 and 2 above), leading from A to B."
  --
  -- and: "For each commodity A, hledger picks a default valuation
  -- commodity as follows, in this order of preference:
  --
  -- 1. The price commodity from the latest declared market price for A
  --    on or before valuation date.
  --
  -- 2. The price commodity from the latest declared market price for A
  --    on any date. (Allows conversion to proceed if there are inferred
  --    prices before the valuation date.)
  --
  -- 3. If there are no P directives at all (any commodity or date), and
  --    the `--infer-market-prices` flag is used, then the price commodity from
  --    the latest transaction price for A on or before valuation date."
  --
  makePriceGraph :: [MarketPrice] -> [MarketPrice] -> Day -> PriceGraph
  makePriceGraph alldeclaredprices allinferredprices d =
    dbg9 ("makePriceGraph "++show d) $
    PriceGraph{
       pgDate = d
      ,pgEdges=forwardprices
      ,pgEdgesRev=allprices
      ,pgDefaultValuationCommodities=defaultdests
      }
    where
      -- XXX logic duplicated in Hledger.Cli.Commands.Prices.prices, keep synced
  
      -- prices in effect on date d, either declared or inferred
      visibledeclaredprices = dbg9 "visibledeclaredprices" $ filter ((<=d).mpdate) alldeclaredprices
      visibleinferredprices = dbg9 "visibleinferredprices" $ filter ((<=d).mpdate) allinferredprices
      forwardprices = effectiveMarketPrices visibledeclaredprices visibleinferredprices
  
      -- infer any additional reverse prices not already declared or inferred
      reverseprices = dbg9 "additional reverse prices" $
        [p | p@MarketPrice{..} <- map marketPriceReverse forwardprices
           , not $ (mpfrom,mpto) `S.member` forwardpairs
        ]
        where
          forwardpairs = S.fromList [(mpfrom,mpto) | MarketPrice{..} <- forwardprices]
      allprices = forwardprices ++ reverseprices
  
      -- determine a default valuation commodity for each source commodity
      -- somewhat but not quite like effectiveMarketPrices
      defaultdests = M.fromList [(mpfrom,mpto) | MarketPrice{..} <- pricesfordefaultcomms]
        where
          pricesfordefaultcomms = dbg9 "prices for choosing default valuation commodities, by date then parse order" $
            ps
            & zip [1..]  -- label items with their parse order
            & sortBy (compare `on` (\(parseorder,MarketPrice{..})->(mpdate,parseorder)))  -- sort by increasing date then increasing parse order
            & map snd    -- discard labels
            where
              ps | not $ null visibledeclaredprices = visibledeclaredprices
                 | not $ null alldeclaredprices     = alldeclaredprices
                 | otherwise                        = visibleinferredprices  -- will be null without --infer-market-prices
  
  -- | Given a list of P-declared market prices in parse order and a
  -- list of transaction-inferred market prices in parse order, select
  -- just the latest prices that are in effect for each commodity pair.
  -- That is, for each commodity pair, the latest price by date then
  -- parse order, with declared prices having precedence over inferred
  -- prices on the same day.
  effectiveMarketPrices :: [MarketPrice] -> [MarketPrice] -> [MarketPrice]
  effectiveMarketPrices declaredprices inferredprices =
    let
      -- label each item with its same-day precedence, then parse order
      declaredprices' = [(1, i, p) | (i,p) <- zip [1..] declaredprices]
      inferredprices' = [(0, i, p) | (i,p) <- zip [1..] inferredprices]
    in
      dbg9 "effective forward prices" $
      -- combine
      declaredprices' ++ inferredprices'
      -- sort by decreasing date then decreasing precedence then decreasing parse order
      & sortBy (flip compare `on` (\(precedence,parseorder,mp)->(mpdate mp,precedence,parseorder)))
      -- discard the sorting labels
      & map third3
      -- keep only the first (ie the newest, highest precedence, latest parsed) price for each pair
      & nubSortBy (compare `on` (\(MarketPrice{..})->(mpfrom,mpto)))
  
  marketPriceReverse :: MarketPrice -> MarketPrice
  marketPriceReverse mp@MarketPrice{..} = 
    mp{mpfrom=mpto, mpto=mpfrom, mprate=if mprate==0 then 0 else 1/mprate}  -- PARTIAL: /
  
  nullmarketprice :: MarketPrice
  nullmarketprice = MarketPrice {
     mpdate=nulldate
    ,mpfrom=""
    ,mpto=""
    ,mprate=0
    }
  
  ------------------------------------------------------------------------------
  
  tests_Valuation = testGroup "Valuation" [
     tests_priceLookup
    ,testCase "marketPriceReverse" $ do
      marketPriceReverse nullmarketprice{mprate=2} @?= nullmarketprice{mprate=0.5}
      marketPriceReverse nullmarketprice @?= nullmarketprice  -- the reverse of a 0 price is a 0 price
  
  
    ]
  